A study on solutions to decrease erosion problems and enhance biodiversity and nature development in the river

River Spui in Winter – Source: zoom.nl

Report

Wageningen University, 2014

Bas Schimmel, Carlette Nieland, Corry Teichmann, Femke Jansen & Larissa Gunst

Final version: 6 June 2014

Client

Name : Delta Talent, ARK Natuurontwikkeling & World Wildelife Fund Company relation : Elma Duijndam, Bas Roels, Gijs van Zonneveld

Phone : 06-20449862 E-mail : [email protected]

Contractor

Authors : Larissa Gunst, Femke Jansen, Carlette Nieland, Bas Schimmel & Corry Teichmann Department : Climate Studies Contact Person : Carlette Nieland Phone : 0623434304 E-mail : [email protected]

Date: 6 June 2014

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Table of Contents

Table of Figures v List of Tables vii Summary viii 1 Introduction 1 1.1 Report outline 1 1.2 The Spui area 2 1.3 Climate Change Impacts 3 2 Methodology 5 2.1 Empirical Framework 5 2.2 Data collection 6 2.2.1 Literature study 6 2.2.2 Interviews 6 2.2.2.2 Definition expert 7 2.2.3 Results of first research stage 7 2.3 Questionnaire 7 2.4 Multi-criteria analysis and sensitivity analysis 9 2.4.1 Criteria 10 2.5 Reliability 12 3 Theoretical background 13 3.1 Importance of biodiversity and natural development 13 3.2 Erosion protection 16 3.2.1 River erosion 16 3.2.2 Protection measures 17 4 Results 20 4.1 Potential erosion protection measures 20 4.1.1 Soft engineering techniques 20 4.1.2 Hard engineering techniques 29 4.1.3 Other techniques 32 4.1.4 Overview (dis-)advantages of potential erosion protection measures 42 4.2 Questionnaire 44 4.2.1 Preferable solutions by stakeholders 44 4.2.2 Arguments provided for solution options 46 4.3 Multi-criteria analysis and sensitivity analysis 51 4.3.1 Standardization/Scoring 51 4.3.2 Weighing 52 4.3.3 Ranking 52 4.3.4 Sensitivity Analysis 54 iii

5 Discussion 55 5.1 Methods 55 5.1.1 Questionnaire 55 5.1.2 Multi-criteria and sensitivity analysis 55 5.2 Results 56 5.2.1 Questionnaire 56 5.2.2 Multi-Criteria and Sensitivity Analysis 58 5.3 General remarks 60 6 Conclusion 62 References 64 Appendix 71 A. Interviews 71 a. Interview Schedule (Dutch) 71 b. Interview and trp Spui – Gerard Litjens - Stroming 72 c. Interview Claus van den Brink – Arcadis 75 d. Interview Marjolein Meerburg & Hans de Bart – Waterschap Hollandse Delta (WSHD) 76 e. Interview Arie Broekhuizen & Ary van Spijk – Rijkswaterstaat 77 f. Interview Jan Kruijshoop – Rijkswaterstaat 79 g. Interview Tom Wilms and Steven te Slaa – Witteveen + Bos 81 B. Questionnaire 83 C. MCA 94 I. Scoring 94 II. Ranking 96 D. Open answers questionnaire (Dutch) 97 Hoe bent u betrokken bij de mogelijke (her)inrichting van het gebied rondom het Spui? 97 Over welke effecten van klimaatverandering heeft u gehoord? 97 Wat zijn naar uw mening de belangrijkste gevolgen van de erosieproblemen? 98 Verbreden van de Rivier: Wat vindt u van deze oplossing? 99 Open : Wat vindt u van deze oplossing? 101 Blokkeren van het Spui: Wat vindt u van deze oplossing? 102 Ground Consolidators: Wat vindt u van deze oplossing? 103 Wilgen matrassen: Wat vindt u van deze oplossing? 104 Wilt u hier verder nog iets over kwijt? 106 Indien werkzaam, voor welk bedrijf werkt u? (niet verplicht) 106 Heeft u nog vragen, op- of aanmerkingen naar aanleiding van dit onderzoek? 107 E. Outcomes of the Multi-Criteria Analysis 108

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Table of Figures

Figure 1: Location of area of interest, including the state of water during the summer drought of 2003. Source: Rijkswaterstaat, 2011, p. 54 ...... 2 Figure 2: Riprap at the shores of the Spui. Source: Bas Schimmel, 2014 ...... 2 Figure 3: Risk of 100-year flood damage for IPCC in Europe. Source: European Environment Agency, 2008 ...... 2 Figure 4: Climate Scenario's for River . Source: IPCC, in lecture: Dr. Fulco Ludwig: Disaster Risk Management Introduction (2014)...... 3 Figure 5: Question from questionnaire: Are you aware of climate change effects in the Spui area? ...... 4 Figure 6: MCDA framework. Source: Linkov et al., 2004 ...... 5 Figure 7: Relation of planning process (rectangle) to multi-criteria decision support framework (bubbles) Source: Males, 2002 ...... 5 Figure 8: Example of MCA question in questionnaire ...... 8 Figure 9: Conservation plan and management types 2014 adjacent to the Spui; Source: Adapted from: Provincie Zuid-Holland, 2011; Natuurbeheerplan, 2011 ...... 14 Figure 10: Photo current situation Spui. Source: Bas Schimmel...... 15 Figure 11: Conceptual model of erosion in Spui...... 19 Figure 12: Brush mattress with live fascine toe protection, profile view (left) and plan view (right). Source: Allen & Fischenich, 2001, page 9 ...... 21 Figure 13: Ground Consolidator element and the use of this concept on the riverbanks. Source: Witteveen+Bos, 2014 ...... 22 Figure 14: The biodegradable geotextiles are pinned into place. Source: Geo- synthetics, 2009 ...... 25 Figure 15: Vetiver grass roots minimizing erosion risks. Source: Holanda & Da Rocha, 2011, Chapter 5.1 ...... 26 Figure 16: Willow Mattress. Source: Salix, 2014 ...... 27 Figure 17: Profile of stone riprap Source: N.C. Division of Coastal Management, 2014 ...... 29 Figure 18: Vegetated gabions: left side vegetated gabion baskets, right side vegetated gabion mattresses. Source: McCullah & Gray, 2005, p. 31, 36 ...... 30 Figure 19: Ecological top layer on stone riprap. Source: ArchDaily, 2008 ...... 30 Figure 20: Upper: Location of the Haringvliet sluices and the Spui. Lower: The dam consistest of 17 sluices and only open when the river discharge is rather high. Source: Rijkswaterstaat, 2008 ...... 31 Figure 21: Dikes near the Spui. Upper illustration for current situation and lower illustration for a possible future construction. Source: Van Winden et al., 2010, p. 22 ...... 32 Figure 22: Sedimentation increase by opening the Haringvliet would stop the erosion and increase biodiversity by sandy soils. Source: Braakhekke et al., 2012, p. 22...... 32 Figure 23: Map of the area with and . Source: Google Maps...... 37 Figure 24: Measures of Room for the River (Dutch). Source: Factsheet Ruimte voor de Rivier, 2007...... 38 Figure 25: Map of the Spui with lines indicated which ground would be used with doubling (yellow) and trippeling (red) of the river. Source: Google Maps, own adaptations ...... 40 v

Figure 26: The Sand engine. Source: Stive et al. (2013)………………………………………………………………………….41 Figure 27: Question in questionnaire: Are you familiar with this solution? ...... 43 Figure 28: Self-ranking of options by stakeholders from questionnaire ...... 44 Figure 29: Self-ranking of options by stakeholders from questionnaire. 1 is high preference, 6 is low preference ...... 45 Figure 30: Screenshot of scoring table in the MCA (BOSDA 3.1). Left: group- and sub criteria; Upper right: top five erosion protection measures in comparison to keeping the current situation; ++ is most positive, 0 menas no impact, -- is most negative ...... 51 Figure 31: Pie charts of weighed main criteria (numbers given in %) determined by six different stakeholder groups ...... 52

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List of Tables

Table 1: Interviewed experts ...... 7 Table 2: Used criteria for the MCA ...... 10 Table 3: Summary of (dis)advantages of erosion protection types ...... 18 Table 4: Overview of advantages and disadvantages of all potential erosion protection measures; Grey background highlights the top five measures ...... 42 Table 5: Self-ranking of options by stakeholders from questionnaire. 1 is high preference, 6 is low preference ...... 45 Table 6: Ranking of top five erosion protection measures as a result of the Multi-Criteria Analysis, compared to the rank of keeping the current situation...... 53 Table 7: Results of Sensitivity Analysis showing distance of an alternative (erosion protection measure) to be ranked first; zero = first rank; numbers in cells of stakeholder groups = distance of first ranked alternative to change to a lower rank; light grey = distance of alternative to be ranked first; dark grey = never ranked first; ...... 54 Table 8: Methods associated with strategies…………………………………………………………………………………………61

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Summary In the South-West of the , the Zuidwestelijke Delta, the estuary of multiple Dutch rivers is situated. The delta includes the river , which has an open connection to the sea, and the Haringvliet which is isolated from the sea water by a large dam. Therefore, the Nieuwe Waterweg contains mostly salt and brackish water, while the Haringvliet contains fresh water. The Spui river connects these two waterways. Due to the tidal influence differences in water level occur between the Haringvliet and the Nieuwe Waterweg, therefore in times of high tide the Haringvliet has a strong pulling force on the water to reduce the water level difference. The volume of water that is needed for this, needs to be transported through the small connecting rivers such as the Spui. Therefore, the flow velocities in the Spui are high. As a consequence the banks and beds of the Spui are suffering from erosion. Erosion could even become more severe due to higher peak discharges from the Rhine with expected climate change. To avoid further erosion, broken rock material, called stone riprap, is deposited on the shores along the Spui. However, due to this measure the biodiversity in these intertidal areas and the space for nature development decline. In this research is investigated how protection of the river banks and beds of the Spui can be done in synergy with nature. A most effective solution is searched for to decrease the erosion problem in the Spui and at the same time enhancing the biodiversity and nature development. A small opening in the Haringvliet (Kier) is considered during the research. In the initial stage literature study was conducted to identify several solution options. A list of 14 solutions resulted to be appropriate for the Spui area, of which 5 solutions were selected based on the objectives of the research, to decrease erosion and enhance biodiversity and nature development. The following 5 solutions were assessed; blocking the Spui, widening the river, implementation of ground consolidators, willow mattresses and opening the Haringvliet. During and after the literature study interviews with experts from e.g. Rijkswaterstaat and Arcadis were held in order get more information and data about, for instance, the effectiveness of a solutions or possible implications for the entire delta. In order to find out which solution would be the most effective a questionnaire was filled in by stakeholders e.g. inhabitants, water boards or government institutions. The questionnaire was used as an input for a multi-criteria analysis (MCA) and to get arguments for or against the selected solutions. The criteria used in the MCA (diverse land use, socio-economic aspects, ecological value, costs and safety) were based on literature study and on interviews with the experts. Scoring of these criteria was based on the same way. Thirty stakeholders, who were divided in groups according to sector, gave their range of importance to the criteria by giving weights which gave an impression of the most preferable solution. The results from the MCA showed a draw of the solution of widening the river and blocking the river. Because blocking the river was not favourable in the arguments given in the questionnaire, we argue the best solution from the MCA is widening the river. The outcome from the directly ranked solutions by the stakeholders, the best solution appeared to be opening the Haringvliet. Since the MCA ranking gives a more scientifically sound result than the direct stakeholder ranking, we conclude that widening the river is the best alternative. After the outcomes of the MCA a sensitivity analysis was performed to check how changing the weights of criteria given by stakeholders could influence the outcome. The results have shown that the groups Inhabitants/Volunteer, Consultancy/Advise and Nature organisations (the more ‘soft’- oriented groups) suggested the same solutions in the same order where widening the river had most potential. The groups Engineering, Agriculture and Government (the more ‘hard’- oriented groups) favoured the solution blocking the Spui. Also we saw that keeping the Spui as it is scored very low for all groups. A striking outcome was that the inhabitants favoured widening the river compared to the government who preferred to block the Spui. In addition, it needs to be considered that a lot of different (often contradicting) arguments and opinions were given in the questionnaire, but also during the interviews. Even though widening the river came out as best solution for this research, the best way of reaching the objectives would be to combine several solutions. An example would be combining widening the river with an opened Haringvliet or with ground consolidators on river banks.

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So, we argue further research is needed in order to investigate the most optimal combination of measures. Additionally, testing this combination of measures to current climate change scenarios is important, too. The time scale of the effectiveness of solutions needs to be taken into account, especially with trends like the lower summer peak discharges of the Rhine and salt water intrusion. The spatial scale of the solution could have large impacts, for example the erosion problem could be solved in the Spui area but can be relocated elsewhere. The weighing of costs on the short and long term scale are also highly relevant. In order to find a most effective solution or combination of solutions an integrated approach is needed, where local support and the involvement of stakeholders especially is very important. Although it is difficult to capture an overview of all arguments, opinions and attitudes towards the ‘soft’, ‘hard’ or ‘other’ erosion protection measures, this research gives a general overview of the preferred solutions by stakeholders. Where different stakeholders with different interests are involved it remains a challenge to find an optimal solution for all parties. Most importantly, this research has shown that there are several opportunities in the Spui area for both decreasing erosion and increasing biodiversity and nature development.

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1 Introduction In the South-West of the Netherlands, the Zuidwestelijke Delta, the estuary of multiple Dutch rivers is situated. Examples of rivers located in the delta are: the Nieuwe Waterweg, which has an open connection to the sea, and the Haringvliet, which is isolated from the seawater by a large dam that includes seventeen sluices. For this reason the Nieuwe Waterweg contains mostly salt and brackish water, while the Haringvliet contains fresh water. The Spui River connects the two waterways and because of the pulling tidal dynamic forces of the Haringvliet large volumes of water are flowing through the Spui. Due to the differences in water level between the Haringvliet (river level) and Nieuwe Waterweg (sea level) the stream velocity of the Spui is very high for such a small river (Mosselman, 2014). As a consequence the banks and beds of the Spui are suffering from erosion. Erosion could even become more severe due to more peak discharges from the Rhine with the expected climate change (Reggiani, P. & Weerts, A.H, 2008). To avoid further erosion, broken rock material, called stone riprap, is deposited on the shores along the Spui. However, due to this measure the biodiversity in these intertidal areas and the space for nature development decline. In this research will be investigated how protection of the riverbanks and beds of the Spui can be done in a more sustainable way, in order to improve the ecological value of the area. In this project the plans of a small opening of the Haringvliet (Kier) are already included. Trends in climate change need to be taken into account so that the solutions are operational in the future.

All together the report will give an answer to the question:

What is the most effective solution for the erosion problem in the river Spui, focussing on synergy with nature by increasing biodiversity and enhancing nature development?

For this question to be answered, the following sub-questions are used: 1. What is in general the current situation of the erosion problem in the river Spui with respect to erosion protection measures and biodiversity? 2. Which solutions currently exist to prevent erosion of banks in Delta Rivers? Are there any other potential innovative solutions available? 3. Which stakeholders should be included, and what is their view on the solutions of the river Spui? 4. Which criteria are the most decisive for the analysis of the most effective option? 5. How to value these criteria most decisive for the analysis?

1.1 Report outline In this report the method used during the research will be described in Chapter 2 and it includes all information about where the survey for the stakeholders was based on. Beforehand some expertise is needed and therefore a few experts were interviewed. Summaries of the interviews can be found in Appendix A. In Chapter 3 more information is provided among the biodiversity and erosion problem in the Spui. The types of erosion in this area, is addressed as well. Chapter 4 deals with all results: the solutions for erosion protection we found and the ones we selected for the survey as well as the outcome. This is done by doing a Multi-Criteria Analysis (MCA) and by open questions. Chapter 5 discusses the outcomes of the MCA, the open questions in the questionnaire and uncertainties within the research. Chapter 6 gives the conclusion for the most effective solution option in the Spui area.

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1.2 The Spui area In this report the focus will be on the problem with erosion, biodiversity and nature development in the Spui area. The Spui is located between the Nieuwe Waterweg/ and Haringvliet and is connecting them (Figure 1). It can get brackish to salty in times of low river discharge or tide. Elevation differences in this area are very small. In Figure 1: Location of area of interest, including the state of combination with the tidal effect, the water during the summer drought of 2003. Source: flow direction of the river changes daily Rijkswaterstaat, 2011, p. 54 (Litjens, 2014).

Most of the surrounded land along the Spui is used for agriculture or a Natura 2000 area (see Figure 9 in the Chapter 3.1 on biodiversity). Some villages close to the Spui are Oud- beijerland, Nieuw-Beijerland, and Middelharnis, which is located across the Haringvliet. A rather big city in this area is , but the Spui is not used for navigation to the harbour of Rotterdam and most boats in this area serve for recreational use (De Bart & Meerburg, 2014; Broekhuizen & Van Spijk, 2014). In the river Spui severe erosion takes place and in certain spots, especially in the corners, the river depth is deeper that 15 meters (Waterschap Hollandse Delta, 2012). If no other measures are taken, the river bottom might erode even further, reaching enormous depths for such a narrow river. At the moment, the riverbanks on both sides of the Spui are protected from eroding by stone riprap (Figure 2). This forms a good basis to stabilize the dikes. In this report a solution to prevent erosion and to increase biodiversity will be searched for. Climate change and its accompanying uncertainties should be taken into account. A good example is the flood risk in this area. In Figure 3 one can see that the risk of flood damage will increase by about 10% in the area of interest. Human-made changes could be of influence as well, such as the ajar decision, meaning that between 2014 and 2018 Figure 2: Riprap at the shores of the the Haringvliet is partly opened and salt water is able to penetrate Spui. Source: Bas Schimmel, 2014 land inwards (Rijkswaterstaat, 2014).

Figure 3: Risk of 100-year flood damage for IPCC in Europe. Source: European Environment Agency, 2008

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1.3 Climate Change Impacts One of the main uncertainties in this research is the natural climate variability and human induced climate change. These uncertainties can be categorized into three possible consequences for the area. The first one is increased sea level rise; because of climate change, it is expected that the world average sea level will rise, however it is uncertain where and when these changes will occur (Rahmstorf, 2007). For instance, the influence of gravitational pulling by ice masses, such as that of Greenland, is uncertain (Spada, et al. 2013). It is therefore difficult to predict how much the sea levels will rise on the coasts of the Netherlands. If sea level will rise, the salt-water can more easily intrude into the project-area, causing more salt-related problems in the area. Consequently, a strategy must be found for these salt-related problems as well. Since the uncertainties are high, this research cannot cope with the impact of sea level rise. The second and third categories are affected by river discharges. The first one is associated with higher discharges than currently exist, while the latter is associated with lower discharges than the situation nowadays. Figure 4 shows the different discharge levels of the River Rhine in the current situation and the expected situation according to the IPCC A2 scenario in 2070-2100.

Current discharges IPCC A2 Scenario 2070-2100

Figure 4: Climate Scenario's for River Rhine. Source: IPCC, in lecture: Dr. Fulco Ludwig: Disaster Risk Management Introduction (2014).

In this graphs, it can be seen that the current, relatively even discharge distribution over the years is changed to a situation with more extremes. Higher discharges are expected in the winter months and lower discharges in the summer months. In the winter, this can result in flooding of the area. The current Rhine River dikes are able to withstand discharges of 50,000 cubic meters per second, while in the modelled ten per cent line, this border is exceeded. This problem is also recognized by the European Environment Agency. In Figure 3 it can be seen that the modelled flood damage for the next 100 years is expected to increase in the South-western delta in the provinces of Zeeland and -Brabant. The Figure shows the projected change in damage of river floods with a 100-year return period between 2071-2100 and 1961-1990. During droughts, the fresh water pressure of water flowing to the sea is declining. Therefore, during high tides, it is possible for salt water to intrude into the estuary and even intrude in the Spui area. During, for instance, the drought of 2003 the Spui turned into a brackish river, preventing the use of the fresh water inlet points Bernisse and Beerenplaat. Beerenplaat is a collecting and storage basin for fresh water. Evides purifies this water to make it suitable for consumption. Evides applies a maximum acceptable chloride concentration of 150 mg/l. Bernisse is an inlet for the northern side of the Spui, distributing water to the canals around the Harbour of Rotterdam (Litjens, 2014). The inlet of Bernisse can guarantee a fresh water supply of 23.000 l/s to its hinterland for water level management, salt flushing and irrigation. It is

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expected that because of climate change drought periods will be longer and more frequent and consequently result in more salt intrusion in the Spui area. Another problem, visible in Figure 4, are the low discharge events. The 2003 drought in the Netherlands showed that adaptation to water shortages considered with Climate Change is necessary (Veraart, et al., 2010). In this situation, as said before, salt water could intrude into the Spui area, causing fresh water problems. According to the A2 scenario, these periods of drought will me more frequent and more extensive, so that salt water can more easily intrude into the Delta. It is therefore important that future projects must be tested on climate change impacts. In the questionnaire, which will be discussed in the next chapters, we asked the respondents if they are aware of effects of climate change (see Figure 5) in the Spui area. 20% answered no, of whom a few were very sceptical on human induced climate change. 80% answered yes, and gave examples on higher water discharges in winter and lower discharges in the summer, warming of the surface with consequences for agriculture, higher sea levels and consequently more salt- intrusion and higher extremes.

20% Yes No 80%

Figure 5: Question from questionnaire: Are you aware of climate change effects in the Spui area?

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2 Methodology In this section we look into the used research methodology. First, we will look at the empirical framework, then the different kinds of data collection are discussed and finally the multi-criteria and sensitivity analysis is explained. 2.1 Empirical Framework The research problem consists of a multi-criteria decision analysis (MCDA; Linkov et al., 2004). Environmental decisions are usually complex because of their multidisciplinary insights; therefore a sound framework is necessary. Figure 6 shows the input for these frameworks; consisting of modelling or monitoring, risk analysis and costs. The modelling is done via the excel sheet and the DEFINITE programme. Risk analysis and costs are not directly used, however they are input for the criteria of the MCDA. The stakeholders’ opinion is used to weigh the different criteria. The Multi-Criteria Analysis, which will be further explained in the next paragraph, handles all these four aspects. The analysis makes it possible to weigh the aspects differently according to stakeholders’ opinion. The article of Linkov et al. (2004) gives an overview of different possible frameworks for MCDA. The Multi-criteria Decision Support Framework seems the most suitable for our specific case. This framework is developed by the US Army Corps of Engineers and is designed for the planning and decision-making of engineering work. The framework also takes environmental factors into consideration. These are being measured in a multi-

Figure 6: MCDA framework. Source: Linkov criterion approach to decision-making (Males, 2002). et al., 2004 Figure 7 shows the framework with in bubbles the steps that are part of the MCDA and in rectangles the planning process considered with these bubbles. The phases of the project are specified in the figure.

Figure 7: Relation of planning process (rectangle) to multi-criteria decision support framework (bubbles) Source: Males, 2002

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2.2 Data collection For the second step of the multi-criteria decision support framework, the inventory and forecast option, alternatives are researched. For this, a literature study was conducted to identify several solution options and possibly even potentially new and innovative ideas can be identified. During and after the literature study interviews were held with experts in order to get more information and data about, for instance, the effectiveness of a solution and the distribution of weights to assign to the criteria in the Multi-Criteria Analysis. At the same time, criteria were explored via the same method: literature study and expert interviews to acquire a general overview. Expert views were also used to evaluate on these criteria. These actions resulted in a decision matrix, the Multi-Criteria Analysis, which is further explained in the next section. After this phase, a questionnaire was sent to several stakeholders in the field to examine their position on the criteria; these positions are the weights for the analysis. The results of the interviews with experts, literature study and the field trip to the river Spui were all included as input data for the Multi-Criteria Analysis. This resulted in the recommendation for the most effective and sustainable option, which is effective in decreasing erosion and in synergy with nature. 2.2.1 Literature study The first step was to find appropriate literature on the subject. Since it is in a specific geographical area, not much academic literature could be found. Therefore, governmental and other reports are used as well for this study. In all cases, the reliability of the reports is checked by researching the outcomes or asking interviewed experts or experts at Wageningen University. Where possible, peer-reviewed materials were used since this is preferable. 2.2.2 Interviews Since data collection via interviews is one of the most important data collection methods for this report, this section will focus on the research methodology on interviewing. Semi-structured interviews are used to gather information from experts (Bryman, 2008). This is an interview in which the interviewer has a series of questions that are in the form of an interview schedule. However it is possible to vary the sequence of the questions. The interviewer also has some latitude to ask further questions to clarify specific responses if it is seen as significant. This flexibility is better for broad questions, such as the research question of this paper. It is important to reduce the error due to interviewer variability; therefore we have constructed a questionnaire. The questions for this questionnaire are broad so that experts can elaborate on it during the interviews. The questionnaire that is used to interview the experts is available in Appendix B. Bryman (2008) states different issues that concern the conduct of interviews. Key is to be well prepared, record the answers in a suitable way and make clear what interviewers can and cannot ask. Therefore, we used two interviewers. One was asking questions, while the other was making notes. A summary of the interview was made and sent to the interviewed experts for them to verify. Where possible and necessary, recording devices were used to record the conversations, this always in consultation with the interviewee. Table 1 shows a table of interviewed persons and their working place. The summaries of the conversations can be found in Appendix A.

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Table 1: Interviewed experts

Interviewed experts Company Claus van den Brink Arcadis Marjolein Meerburg & Hans de Bart Waterschap Hollandse Delta

Tom Wilms, Steven te Slaa Witteveen+Bos Arie Broekhuizen & Ary van Spijk Rijkswaterstaat

Jan Kruishoop Rijkswaterstaat

Gerard Litjens Stroming

2.2.2.1 Definition stakeholder The term ‘’stakeholder’’ is used to define someone who is directly involved in or around the area of the Spui, like for instance an organization, which is responsible or shares responsibility for either maintenance or design of new project, plans. It also defines people involved in policy planning of the area, inhabitants, farmers and experts, which have relevant knowledge on the development or erosion problems of the Spui. The stakeholders filled in the questionnaire when they have not received information about the project beforehand. This is because this might have influenced the answers given. Stakeholders are for instance from e.g. Rijkswaterstaat, Arcadis, Deltares, farmers, inhabitants nearby the area of the Spui, the water board Hollandse Delta, Ark Natuurontwikkeling, WWF, the province and others. It is important to note that stakeholders from for instance RWS and Arcadis, were not interviewed.

2.2.2.2 Definition expert An expert is different from a stakeholder in the sense that they have received information about the project and because of their expertise and knowledge about the project (erosion/biodiversity/nature development). By doing interviews information was gathered for further study and recommendations of experts were often used as a guideline for making the report. The experts gave direction or guidance to come up with new solutions. A summary of the interviews that are held is given in Appendix A. A list of the interviewed experts is also listed under acknowledgements. 2.2.3 Results of first research stage The results of this first research stage were the thirteen potential solutions in our long list, which ended in the five options of our short list. Furthermore, criteria on which the solutions could be scored were determined. This was done through the interviews and literature study; relevant solutions and criteria were asked. Every time, these two outcomes were checked by multiple experts. In the end, the project group decided which solutions were the best to include in the shortlist. We have used all different criteria that were mentioned during the conversations, since they were in our view all important. These criteria needed to be operationalized further; in the next chapter this will be explained in more detail. 2.3 Questionnaire During the project, a questionnaire was sent to different stakeholders who are involved in the river Spui area. This questionnaire was sent in Dutch as all of the stakeholders live in the Netherlands. Important stakeholders and/or actors are Rijkswaterstaat, Waterschap Hollandse Delta, Deltares, Evides, WWF, ARK Natuurontwikkeling, Natuurmonumenten, province, municipality, and more in general industry, inhabitants and farmers etcetera. The questionnaire was constructed in Google Docs, in which forms can be made according to an own template. A 7

major advantage of Google Docs is that it automatically gives summaries of the completed questionnaires. It also gives a possibility to export the outcomes to an excel spreadsheet in which comparisons can be made. The questionnaire consists of four sections with in total nine main questions. The sections are put in a specific order to minimize the risk of probing. Probing is the steering of an answer to a specific direction by, for instance, earlier given information (Bryman, 2008). The questionnaire took circa 10 to 15 minutes to complete. The first section, consisting five questions, was used for the completion of the Multi- Criteria Analysis. In each question, multiple options, criteria or values were given for the future situation of the Spui in comparison to the situation as it currently is. The question was to divide 100 points over these options, criteria or values. The option that was seen as most important got the most points and the least important option received the least points. In Figure 8 an example of this question is shown. The other questions are available in Appendix B. In this part of the questionnaire, the respondents are not yet familiar with the proposed solutions to prevent them to fill out the criteria while they are thinking about the solutions. The questions six to nine were not used for the Multi-Criteria Analysis but to provide arguments and insights for the report. These questions can be found in Appendix B, in Dutch. The question numbers in the text correspond to the question numbers in Appendix B. The first question (6) of this section of questions poses different questions on the area of the Spui, these are all open questions to ask the respondents on background information on the Spui area to see how familiar they are with this area; this question can, if necessary, later on be used to filter on knowledge of the Spui area. Question 7 is about the different solutions; a short summary of each solution is given with some background information and a picture; these solutions are numbered 1 to 5 in the questionnaire. This information is, however, limited to prevent probing. For each solution it is asked if one is familiar with this method. Furthermore, it is asked if one can mention positive and negative impacts of this solution. In question 8 we ask the respondents to rank the solution according to their preference, it is also asked. Question 9 is composed of a few multiple-choice questions. These questions are used to give us some background information on the respondents so that we can relate this personal information to the given answers on the prior questions. These last questions can be used for grouping stakeholders for the Multi-Criteria Analysis. This will be explained further on. As said before, the specific order in the set-up of this questionnaire is done in this way to prevent probing from other questions. The questions are first relatively broad and not directly focused on the solutions and later on specifically focused on these solutions.

Figure 8: Example of MCA question in questionnaire

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2.4 Multi-criteria analysis and sensitivity analysis Multi-Criteria Analysis (MCA) was used to firstly assess different sustainable solution options preventing soil erosion in the Spui and secondly find the best solution to answer our main research question. Beinat & Nijkamp (1998) state that MCA is especially useful to approach complex decision problems where several criteria need to be taken into account. It provides a way to structure a decision problem, to explore the concerns of decision actors and to evaluate alternatives under different perspectives. To apply MCA the dutch version – BOSDA 3.1 – of the decision support software DEFINITE (DEcisions on a FINITE set of alternatives) was used. According to Janssen & Herwijnen (2006), firstly several options (called alternatives) need to be defined to set up an effects table. In our case these alternatives were the determined top five sustainable protection measures against soil erosion, increasing biodiversity and nature development. Furthermore, a sixth alternative was added, which is to keep the current situation of the Spui area. Subsequently, these six alternatives were compared by a set of chosen criteria, to which scores can be assigned. To find the most important criteria for our project, further literature investigations and also expert interviews were done. Finally, five group criteria and corresponding three to six sub criteria were determined. In the following chapter these criteria such as “Ecological value” and the reasons for choosing them are more precisely described. The consultation with experts and the literature research were additionally used to assign scores on a --/++ scale (minimum/maximum range) to the defined criteria. Further three steps then needed to be taken (Janssen & Herwijnen, 2006): 1. Standardization, 2. Weighing and 3. Ranking. Criteria standardization is done to make the scores comparable. Within this project maximum standardization was used, where the worst score (--) received a value of zero, the best value (++) one and thus all values were divided by the maximum value. The next task was to weigh criteria as not all are equally important. The weight given depends on the specific aim and on the involved stakeholders. In our case, many different stakeholders were asked within an online-survey what they value the most or least by dividing 100 points over the (sub)criteria (see Chapter 2.3 Questionnaire). If, for example, one stakeholder prefers sustainable solutions and increasing natural development and biodiversity as well as flexibility and dynamics in ecosystems, higher weight was given to the related criteria. Thus, for instance in the group criterion “Diverse land use” 50% of the weight was assigned to the sub-criterion “Nature” and the remaining 50% of the weight were distributed evenly among “Recreation”, “Navigation”, “Agriculture”, “Housing” and “Drinking water supply”. Weight was not only assigned to sub criteria, but also to group criteria. Subsequently, the third step - ranking - was done by weighted summation and comparison of final scores. Finally, the highest score gives the best alternative for the specific stakeholder preference. Within the Spui-project not all stakeholders were considered solely, but in stakeholder groups as this helps summarizing and simplifying peoples opinions. These six stakeholder groups were built according to sectors people are acting in: Consultancy/Advice, Agriculture, Engineering, Inhabitant/Volunteer, Nature Organisation and Government. Their given weights in the questionnaire were averaged and then entered into the MCA. Thus, the end result showed for each of the six stakeholder groups the ranking of the top five sustainable erosion protection measures in comparison to the rank of keeping the current situation of the Spui. After conducting the MCA the reliability of the result was verified via sensitivity analysis of given stakeholder weights. This was done with the support software BOSDA 3.1 as well, to test how robust the ranking of alternatives is for uncertainties in given weights. It might be the case that already small changes within these values result in a different ranking and hence a different best alternative (Janssen & Herwijnen, 2006). As observing any changes on the ranking of all alternatives would have been too time consuming for the limitations of this report, the focus was just laid on the robustness of the best (top) alternative. Hence, it was searched for the shortest distance of weight when the best alternative is not on the first rank and subsequently for the alternative which would be ranked first instead. All these results from BOSDA 3.1 were compared to the stakeholder answers given to the open questions in the second part of the questionnaire.

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2.4.1 Criteria In the following table (Table 2) all used group- and sub-criteria are described.

Table 2: Used criteria for the MCA

Group criterion Sub-criterion Description Diverse land use “Land use is characterized by the arrangements, activities and inputs people undertake in a certain land cover type to produce, change or maintain it. Definition of land use in this way establishes a direct link between land cover and the actions of people in their environment.” (FAO, 2005) Recreation - Land used to do leisure activities Navigation - Rivers within the land with shipping activities (ferries, cargo ship, etc.) Agriculture - Land used for farming Housing - Land used as living area Nature - Land used for naturally developing ecosystems (grasslands, mires, forests, etc.) Drinking water - Land used to provide clean drinking water supply Socio-economic criteria Socio-economic criteria comprise acceptance of direct (economic) benefits (or damage) for humans from the erosion protection measure itself. Esthetics - Beauty of the erosion protection measure Local support - To what extent the new plan will be accepted by stakeholders Landscape - All landscape features of a non-economic value value such as clean water and presence of plant / animal species. Economic value - All landscape features of economic value such as agricultural land and recreational facilities Ecological value The ecological value is defined as the level of benefits that the factors, which make up natural ecosystems such as water and biota, provide to support native live forms. Both humans and nonhumans can profit from ecological values. Ecological value is greatest when ecosystems are in their most natural state as they then reach their peak of natural health and their greatest level of native life support. (Cordell et al., 2005) Increase - Enlarge biodiversity due to installation of erosion biodiversity protection measure (for definition of biodiversity see Chapter 3 “Importance of biodiversity and natural development”) Stop salt - Prevent oceanic salt water from flowing land intrusion inwards (groundwater) and/or river upstream due to installation of erosion protection measure Restore tidal - Get regularly changing high and low tide back in dynamics the river systems Prevent - Prohibit removal of soil and rocks by water erosion, flowing over banks or streaming in the river promote - Support erosion protection measure where sedimentation eroded material is accumulated Costs Costs in this case describe expenses related to the erosion protection measures. Installation - Initial costs as a result of installing the erosion costs protection measure 10

Longevity of - For how long an erosion protection measure will solution endure after initial installation and the accompanying costs Maintenance - Accruing costs as a result of maintaining the costs erosion protection measure Safety - How secure an erosion protection measure is considering for example the reliability of dikes

All the solutions we are going to use consider the main question, therefore all solutions are expected to be in synergy with nature and prevent erosion. These different criteria are based on the report of Rijkswaterstaat (Dekker & De Zeeuw, 2012). However, since the main focus of this report is on environmental quality, we have adapted the criteria used for the MCDA. The main criteria used by Rijkswaterstaat are costs, usability and safety and in a lesser extent land use and ecological value. The criteria safety could be used instantly; further specification was not necessary for this assignment. We decided to specify costs, since, according to the experts we have interviewed, long-term solutions were important. It seemed therefore not right to solely consider installation costs; that is why maintenance costs and longevity were also added as a criteria. The usability criterion was also adapted. Usability is the way in which a specific solution proofs its use, such as for a waterway, the possibility to navigate it. We have decided to group this under different users of land, which are now present in the area. In that way, stakeholders can give their preferred land-use for the Spui area. The options we considered to be currently present in the Spui area and also possible for in the future are recreation, navigation, agriculture, housing, nature and drinking water supply. This land-use now plays an important role in the Spui. The last criterion we have adapted from Rijkswaterstaat is ecology, however this does play a lesser role in decision-making with the agency. Therefore, there is decided to add sub- criteria on ecological value. These are all based on the interviews with expert, that almost all mentioned the importance of the following criteria: change in biodiversity, salt-intrusion problems, tidal dynamics in the river and the prevention of erosion or the promotion of sedimentation. These four criteria were then grouped in ecological value. Experts also mentioned the importance of local supports for plans. They mentioned that if the area after the project looks better, it could be higher appreciated. However, this is not always the case. Therefore, it is decided to include both criteria as well, in a socio-economic cluster. Landscape value is also important in this one, because it gives information on non- economic value such as clean water and presence of plant / animal species. This could improve the quality of life and can be attractive for recreation. Landscape value is more connected to and has correlation with the ecological value of the area. The last criterion, economic value, is again adopted from Rijkswaterstaat, and has a correlation with the usability aspect of the plan. In the first set of criteria, only space for navigation, agriculture etcetera was considered. The economic value also looks into the economical side of land-use and is therefore more connected to land- use. The scoring of these criteria will be discussed in Chapter 4.

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2.5 Reliability Reliability is another important aspect of scientific research. Since some of the data collection is based on qualitative research, reliability could be an issue. This is especially the case with data collection versus shareholder consultation. On the basis of the framework, data is collected, according to the described process in Chapter 2.2, and this is used to make the scores in the MCA. Literature review and data collection from the interviews with experts are the two most important data sources. It is important that experts, who will give information on the scientific part of the project, and stakeholders, who have a stake in the projects, are different persons. Sometimes, however, these two could overlap. In our research, we have excluded the stakeholders that were also experts. It is important to distinguish the experts and the stakeholders to create more reliable outcome. In the results of the questionnaire, we have taken a close look on who was an expert and a stakeholder as well. No problems were found for the use of the questionnaire data.

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3 Theoretical background This chapter elaborates on definitions, which are used for biodiversity and erosion. The importance of the conservation of biodiversity is discussed as well as the need for nature development. In addition an explanation is given concerning the difference of ‘soft’, ‘hard’ and ‘other’ protection measures.

3.1 Importance of biodiversity and natural development As explained in the introduction and current situation of the Spui area (Chapter 1), the hard measures in the Spui like hard rock or stone riprap decline biodiversity in this area. The hard measures reduce the space for natural development and therefore the area has a lower ecological value. In this report the term ‘biodiversity’ is meant as in Article 2 (CBD, 1992) "Biological diversity means the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems’’. In other words, all organisms and species, their habitats and their interactions determine the biodiversity. WWF (2014) divides biodiversity in the following three definitions: genetic diversity, species diversity and ecosystem diversity. In the case of the Spui, increasing biodiversity could mean an increased ecosystem diversity and enhancement of different species in the area. In addition, landscape dynamics or structural diversity and the different functions in the ecosystem (ecological succession) are components, which also determine biodiversity (Ward and Tockner, 2001). The definition of sustainability is meant in this report as the ecological component of sustainable development; ‘A capacity of ecosystems to maintain their essential functions and processes, and retain their biodiversity in full measure over the long-term’ as used in the Business dictionary (2014). Currently, because of hard measures like stone riprap, more habitats are lost and therefore less species occur in the area. In order to enhance the biodiversity habitats need to be improved, especially vegetation growth in some cases and/or more natural space. The current fresh water ecosystem in the Spui is surrounded by Natura2000 areas (Litjens, 2014). These areas are important since the goal of the Natura2000 projects is to increase ecological values and to improve biodiversity. When looking at the area surrounding the Spui, for instance the Old (Oude Maas), the area could have an important potential for several (new) habitat types or for the conservation of current biodiversity. The conservation of the habitat of the Northern vole could be important, as explained in Rijkwaterstaat (2014), as well as the extension of the quality of habitats. This means for instance if the roughness of certain areas along the riverbanks is increased, the Northern voles have more places for shelter and the population can grow. In the law on nature protection (Natuurbeschermingswet 1998 in Rijkswaterstaat, 2014) it is stated what the ecological values are for the Natura2000 area. This law describes the aim for conservation of habitats, species and specific bird types. Litjens & Maka (2010) explain how climate buffers in the area of the Spui could also contribute to the increase of biodiversity, by creating more space for nature development. So, there are opportunities in the area for biodiversity improvement. To be specific on the Spui, several riverbanks are covered in grass and certain types of reeds and other parts of the Spui are just covered with for instance stone riprap. There could be opportunities in developing more habitats here, by using different erosion protection measures than currently present. Different erosion protection measures are explained in 4.1. A broad view on the current situation of the Spui concerning biodiversity and nature development is shown on a map of the Spui with different vegetation types, see Figure 9 (Natuurbeheerplan, 2011).

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Figure 9: Conservation plan and management types 2014 adjacent to the Spui; Source: Adapted from: Provincie Zuid- Holland, 2011; Natuurbeheerplan, 2011

This figure illustrates 10 different types of vegetation and land use. A couple of most apparent vegetation types are mentioned below. Most of the nature in the map is man-made, but do have ecological value (see definition in 2.4.1. Criteria). The Hornbeam and ash forest (Haagbeuken-en essenbos, N14.03) have relatively rich vegetation structure types and a notable spring flora. Also planted willows and poplars occur in this forest type, which can be important for certain amphibians, colonizing spore plants and birds. The herb- and fauna-rich grassland (Kruiden en faunarijk grasland, N12.02) is common in many landscape types. This type of land use has been declined for several decades due to increased agricultural practices. The quality of this grassland type is determined by a high variety of vegetation structure and herb-rich grassland with a lot of (small) fauna. However, special or rare species do not often appear in this type of grassland, but they are important for less rare species, for instance butterflies and other insects and birds. The moist meadow bird grasslands (Vochtig vogelweidegrasland, N13.01) are important for the diversity in species. Due to agriculture and livestock the meadow bird population has decreased, since the preferred area changed in a negative way for the meadow birds, e.g. by predators or a lesser quality of grassland. Internationally, these birds are a special species and the Netherlands has the responsibility to conserve meadow bird populations. In the river and floodplain landscape (Rivier en moeraslandschap, N01.03), species like the sea eagle (zeearend), red deer (edelhert) or beaver are typical for this type of landscape (Natuurbeheerplan, 2011). These species are important for increasing biodiversity in the area. For this report one focus lies, as said earlier, on the improvement of biodiversity, but not on describing exactly on which species or habitats are aimed to enhance. This description above of the most common vegetation types in the area is useful for a general impression of the area and might help understand the current situation of the Spui. Pictures of the Spui are shown in Figure 10 on the next page.

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Figure 10: Photo current situation Spui. Source: Bas Schimmel

In other parts of the Netherlands the importance of biodiversity and nature development is also evident. The ‘Building with Nature’ concept describes the possibility of using natural processes or forces to create opportunities for nature while developing infrastructure (Ecoshape, 2012). This approach is used as a base for our report: to optimize the natural forces and development in searching for a solution to the erosion problem, on which will be elaborated in 3.2. The problem of increased erosion has an effect on nature development. Nature development in this report means the amount of vegetation growth that can occur in the area concerning the requirements that it needs to establish or develop. When sediments are removed or deposited further in the area vegetation growth can be inhibited. The riverbanks can be less stable when eroded, as will be explained in the next section, creating a problem for vegetation, which needs time to establish an initial strength to survive and not get washed away by water immediately. Therefore, when the erosion problem is tackled it becomes beneficial for nature development and possibly for the creation of habitats for several species (SEPA, 2008). The current use of stones hampers nature development and only diminishes the erosion problem. The reduction of erosion could go in line with nature development and improvement of biodiversity if other measures or solutions are implemented. However, a lot of aspects need to be considered. Especially, since there are many ways in which biodiversity can be improved, as explained further in the report. There are several examples given of solutions, which are combinations of measures that reduce erosion and enhance nature development, like for example planting willows and implementing an ecological top layer. In general is described the advantages and disadvantages for biodiversity and the contribution to ecological value.

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3.2 Erosion protection This chapter will give some information on general erosion protection measures. 3.2.1 River erosion The main processes occurring in rivers are erosion, transportation and sedimentation, which make the river a dynamic system. River erosion is the removal of soil particles from the riverbed and banks due to the force of the water. When rivers are subject to an increase in flow depth and velocity, the increase in energy of the river causes erosion to affect both riverbed and riverbank making the river deeper and wider (Iowa Department of Natural Resources, 2006). These are the four main ways in which the river shapes the land surface (Van Netten, n.d.):

Hydraulic action This is mechanical weathering where loose particles from the banks or the riverbed are being removed and transported by the flowing water. Abrasion/scouring Particles, which are carried in the water, have the ability to abrade the riverbed and banks. Attrition Stones, which are carried in the water, can break into smaller pieces by the collision between the particles themselves or between the particles and the riverbed. This process will be more efficient in rivers with high flow velocities. Solution Minerals from rocks can be dissolved into the water. This can cause major gaps in the case of for example limestone.

River erosion in delta areas has some specific characteristics. In delta areas the sea influences erosion and sedimentation processes due to highly fluctuating water levels as a consequence of tidal currents. Currently there is a deficiency in sediment in the ZW-Delta because there is no supply from both the sea and rivers (Deltaprogramma | Zuidwestelijke Delta, 2010).

Factors that are influencing the process of bank erosion are typically (Geotalk, 2012).:

Flow dynamics Discharge, velocity and turbulence Material composition Grain size, compaction, texture, vegetation and stratification Channel geometry Width, depth, height and slope of bank, curvature and gradient Human-influenced Surge barrier, navigation and dams

3.2.1.1 Riverbank vs. riverbed Two types of riverbank erosion are bank scour and mass failure. Bank scour is the direct removal of bank material as a result of the flowing water and the sediments suspended in it. Mass failure is characterized by sections of the bank sliding or falling into the river, leaving bare and near- vertical banks as a result (University of Victoria, 2008). Erosion of banks is in general enhanced by overgrazing by cattle, by recreation by use of powerboats and walking paths damaging the surface and vegetation and by mismanagement of trees on the banks, which can be undercut by the river leading to unstable situations of the banks (The Wildlife Trusts, 2001). Riverbed erosion is caused by a large volume of water flowing with high velocity through a river, which leads to incision of the river. The erosion rate of the riverbed can increase dramatically when easily erodible material, e.g. sand, is reached after a more difficult erodible layer, e.g. clay or peat, is eroded away. Then this process can cause development of deep holes in the river which afterwards can extend outwards reaching the banks of the rivers within several weeks to months. This will lead to scouring of the submerged parts of the banks, which make them instable and susceptible to collapsing (Mosselman, 2013). It should be taken care of that scouring of the riverbed could undermine the riverbank protection. Therefore, it is necessary to incorporate toe protection of the bank to avoid this problem. 16

3.2.1.2 Erosion in the river Spui Since the Haringvliet has been closed off from the North Sea, a redistribution of flow in the South-western Delta was caused. The resulting increased flow in the Spui has resulted to deep holes in the riverbed because of the eroded shallow clay layer. In other locations the clay patches are deeper and it takes more time for the river to cut through this thicker clay layer. Together this caused non-homogeneous erodibility of the riverbed in the Spui. This process occurs in a time span of weeks to months and it progresses fast and will threaten the banks and dikes as the holes extent outwards therefore causing the banks to become instable (Mosselman, 2013). The banks are not only threatened indirectly by the erosion of the riverbed, but also directly by the water flow. Reducing the tidal movements would reduce this erosion. For the Spui area it is acceptable to have some erosion there as it is part of the natural estuarine dynamics of the area. Currently the riverbed and banks are protected by stones that are deposited there by Rijkswaterstaat who is responsible for the riverbed and the depth of the river, and by Waterschap Hollandse Delta who is responsible for the bank protection and the dikes (De Bart & Meerburg, 2014).

3.2.2 Protection measures Rivers are often not allowed to follow the natural course as there is an increasing demand on space especially in delta areas. Humans have intervened in the natural dynamics of rivers using engineering techniques to change the course and to protect the bed and banks from eroding. There are multiple protection measures available and we divided them in three main types: Hard engineering techniques, soft- or bioengineering techniques and ‘other’ techniques. The first two types are treating the symptoms of erosion and the latter one includes measures that address the causes of erosion.

3.2.2.1 Hard engineering Hard engineering techniques involve the construction of artificial structures that are imposed on the river and use force to control the river. Usually they are very effective and will provide immediate results, but at the same time they tend to be disruptive as well, harming the ecological system and its natural processes. Due to the reason that the solutions are forced upon the system, the consequences when the technique fails are usually worse than in the case of soft engineering techniques. Also high costs for installation and maintenance are involved in hard engineering solutions and therefore it is not always feasible (Jackson, 2012). However, hard engineering have proved to be often successful and therefore they are often applied as they are considered as a ‘safe’ option (Van den Brink, 2014). But in the end it can result in even worse conditions than before the installation of the hard measure due to, sometimes unexpected, changes in the natural processes (Jackson, 2012).

3.2.2.2 Soft engineering The main aim of using soft engineering techniques is to incorporate ecosystem functions and natural processes to guarantee a sustainable solution for the erosion protection of riverbanks (Deltares, 2012). Soft engineering techniques involve the use of live vegetation and woody and herbaceous material for bank stabilization. An increase in strength and structure of the soil is accomplished by a dense network of roots leading to reinforcement of the soil. Besides the increase in strength and structure of the soil, also the aboveground vegetation lowers the erosive potential of the riverbank. This is accomplished by increasing resistance to the water flow and reducing flow velocity at the soil surface. In addition, the aboveground vegetation is able to prevent part of the scouring effect and can even trap sediment (Bentrup & Hoag, 1998). Bioengineering projects are considered to be less expensive with regard to construction, materials, labour activity and maintenance over time in comparison to the hard engineering 17

solutions. They require more time to establish, but once they are established it will often be self- maintaining and re-generating. There are also some constraints in this type of protection as in some cases the vegetation fails to grow and the plants can be subject to scouring. Other limitations are that the vegetation can be damaged by freezing and thawing and by debris or it can be destroyed by cattle stepping on it and using it to graze. The maintenance is important, especially in the initial phase of the construction, and therefore it may take quite some effort in the beginning for the protection measure to well establish (Garanaik & Sholtes, 2013).

3.2.2.3 Other techniques With other techniques we refer here to measures that are not per se focused on protection against erosion, but on tackling the cause of the erosion. For instance more room for the river, blocking of part of the river or in the case of the Spui: opening of the Haringvliet sluices. When the cause of the erosion is eliminated, it might result in less protection measures needed or it may give the opportunity for other, less hard, types of protection measures which are more in synergy with the natural system to be used; e.g. multi-functional dikes. This will create opportunity for more gradual transitions between water and land and also more chances for nature development (WNF, 2010). When riverbank erosion will be prevented using ‘other’ techniques, riverbed erosion will most likely be reduced as well. These types of measures probably are only needed when in the past the natural river system has been adjusted and controlled to the need of humans. Otherwise the system would by itself find the most efficient way to flow and cut through the landscape. Therefore, it might be best to return to the original situation, but often this is not possible or feasible. The suggested measures involve drastic changes in the area around the river and it would therefore need the support of the communities involved.

In Table 3 a summary is provided of the (dis)advantages of the three types of erosion protection measures.

Table 3: Summary of (dis)advantages of erosion protection types

Advantage Disadvantage Hard engineering Often successful Costly

Effective especially for high flow Disrupting ecosystems velocities Could worsen the situation

Soft engineering Incorporates natural processes Needs more time to establish

Less expensive Vegetation can get damaged by freezing/thawing and by debris Mostly self-maintaining and re- Chance that vegetation fails to generating grow Other techniques Tackling the cause of the erosion Involves drastic changes to the area Support of community needed

3.2.2.4 Overview of erosion and protection measures The on-going processes of the Spui discussed before are depicted in the conceptual model in Figure 11 The three types of protection measured mentioned above could be used to tackle the erosion of the riverbed and the riverbanks. The soft- and hard engineering techniques will only decrease the erosion of the riverbanks as it will only tackle the symptoms of the erosion. The

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‘other’ techniques will tackle the cause of the erosion, therefore reducing erosion of both the banks and the bed. When measures are taken to reduce the erosion of the riverbanks, there will be more opportunity for nature to develop and for biodiversity to increase. This will especially be the case when protection measures are taken which will use natural processes.

Figure 11: Conceptual model of erosion in Spui

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4 Results This chapter will discuss the results of the different analyses. First, fourteen potential erosion measures will be discusses. Out of these fourteen, five will be chosen fort he project area. These five will be discussed according to the results of the questionnaire (4.2) and the Multi-Criteria Analysis (4.3) 4.1 Potential erosion protection measures By doing literature research and interviewing experts a list of fourteen potential ways could be compiled on how to solve the erosion problem and at the same time increase biodiversity and/or nature development in the Spui (see below). This list contains techniques treating the symptoms of riverbank erosion (hard and soft engineering techniques) as well as addressing the underlying causes of erosion (large scale measures). For all of the techniques a description is provided as well as the advantages and disadvantages. Table 4 in Chapter 4.1.4 gives an overview of the latter. Additionally, it is discussed whether the introduced technique is sustainable and if it is likely to achieve an increase in biodiversity and nature development in and close to the Spui. Five of the fourteen erosion protection measures, which have most potential as a solution in the Spui area, are more extensively looked at. They are the most sustainable and the most likely to achieve an increase in biodiversity and nature development in and close to the Spui. Out of the fourteen potential measures we selected willow mattresses in combination with toe protection, Ground Consolidators, blocking the river, widening the river and opening the Haringvliet.

Soft engineering techniques:  Willow mattresses in combination with toe protection  Ground consolidators  Geotextiles  Vetiver System  Planting willows  Clay and sludge Hard engineering techniques:  Stone riprap  Vegetated gabions  Bio-cement  Ecological top layer Other techniques:  Widening the river  Blocking the Spui river  Opening Haringvliet  Sand engine

4.1.1 Soft engineering techniques As already explained the main aim of using soft engineering techniques is to incorporate ecosystem function and natural processes to guarantee for a sustainable solution in protecting riverbanks against river erosion (Deltares, 2012). Under this heading we include applying willow mattresses and ground consolidators, the usage of biodegradable geotextiles, the Vetiver System and planting willows.

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4.1.1.1 Willow mattresses in combination with toe protection Brush mattresses are made out of thick layers of cut branches, covering riverbanks to stabilize the slopes. To work effectively the slope should not exceed 1:1, at best it would be 1:3 (European Commission, 1998). In order to initially secure the mattresses from moving, wooden stakes and nets are used to hold everything in place. For the mattresses in particular live willow branches are suitable as they have the ability to shoot, root and grow in wet soil conditions. Hence, they rapidly develop into thick and functional riparian vegetation. The mattresses will increase the surface roughness and thus decrease flow velocities near the banks, which can lead to reduced erosion rates (McCullah & Gray, 2005). Although this solution provides already immediate protection, when the live willow mattresses will rapidly shoot, root and grow, after one or two years the layer will be thicker which improves mechanical stabilization of the bank even more. The openings and gaps in the mattresses allow effective sediment as well as seed trapping. Due to the structure it provides a habitat and shelter for wildlife such as birds, small fur-bearing animals and insects that on their turn are fed upon by fish. The habitat for fish improves when the vegetation has reached a sufficient height for providing enough protection from predators. (Allen & Fischenich, 2001; Connecticut River Joint Commission, 1998). In between the willows other vegetation is likely to grow, therefore enhancing biodiversity (Environment Agency, 2010). Additionally, other trees can be added to the cover such as alders and ash-trees. Before deciding to choose the mattresses as erosion protection measure the consequences of failure should be considered as well. What will for instance happen if the mattresses are infected by a disease or when it is affected by an insect infestation? (Allen & Fischenich, 2001). The use of willow mattresses can well be combined with other toe protection measures to prevent undercutting of the mattresses at the base of the riverbank. Fascines, living or non- living, can be used for this purpose. These are bundles of (willow) branches, which are placed at the bank toe and can be secured by wooden stakes as well. Making use of live branches can be a long-term option as they will root and grow, which assures stable banks on the long term. Another option for toe protection is the use of rock at the base. Furthermore, the mattresses should be extended up the bank far enough as well to prevent scour above the construction to develop in situations of high water levels. The installation of the mattresses is labour intensive and requires a large amount of material, which should preferably be collected from nearby locations. However, the total costs are not high and therefore this concept is economically beneficial. Costs according to Allen & Fischenich will be around €25/m2 including construction, maintenance and contractor costs. When implementing the willow branches should be placed closely together over the whole slope with the branches cuttings inserted into a trench at the bottom of the slope just below the toe protection (see Figure 12). Good soil-stem contact is needed and the slope should therefore have a regular shape. The branches should be directed at a slight angle in the direction of the stream flow. To tie the branches to the wooden stakes wire or coconut rope is used after which it is covered by a 3-4 cm thick soil layer. The toe protection is installed up to the low water level. In order to create optimal condition for fast root development the implementation should be done in autumn. Until vegetation becomes well developed repairs probably are needed and inspections after high stream flow events or floods are needed for the examination of undermining and scour. After the vegetation is well established maintenance is greatly reduced (European Commission, 1998; Allen & Fischenich, 2001). In Germany the use of living brush mattresses has been successfully used already in a few rivers as a test. It was tested in the Rhine, Weser, Main and Saar rivers (Bundesanstalt für Gewässerkunde & Bundesanstalt für Wasserbau, 2012).

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Figure 12: Brush mattress with live fascine toe protection, profile view (left) and plan view (right). Source: Allen & Fischenich, 2001, page 9

The slope of the banks along the village of Nieuw-Beijerland is 1:2.6 according to Zwanenburg et al. (2013), which would be suitable for the willow mattress construction. However, in a report investigating the risk for shear of the banks it is mentioned that along the whole Spui the slope is at least steeper than 1:4.5 and that on many locations the slope even exceeds 1:1 (Lee et al., 2012). In the latter case the willow mattresses would not be a good option. So further research needs to be done on whether the slopes are not too steep in order to successfully implement this concept. Besides the requirement for slope, also stream flow should be considered. The stream flow should be sufficient for the plants to grow, but is should not exceed the speed that is tolerated by the vegetation. In addition the water level should be high enough to provide enough moist to the plants during the growing season and preferably sunny locations should be chosen (Allen & Fischenich, 2001). The Spui typically has a stream flow of 1.4 – 1.9 m/s (De Bart & Meerburg, 2014) which does not exceed the critical speed of 3.7 m/s (Allen & Fischenich, 2001) for which the mattresses are not able to sustain. In that respect the Spui is suitable for the installation of mattresses. The Spui is a perennial river, although with tidal effects, and thus the plants will have sufficient moisture available to develop. On the robustness of this solution regarding climate change further research should be done. When salt water will reach the Spui area more often and the water therefore becomes brackish, the willows will not be able to survive (Tangelder et al, 2013).

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4.1.1.2 Ground Consolidators Ground Consolidators (GC’s) are used in the biodegradable shoreline protection approach, or shortly B-shore approach, to develop a degradable bank protection. GC’s can be made out of degradable and non-degradable material. The latter version can be used for the parts of the riverbank that are always submerged to make sure the toe of the bank will remain protected. The GC elements can have various sizes depending on the specific circumstances and together they form a mat of clustered wire-shaped elements, which is put on top of the bank (see Figure 13). The formed mat is supposed to prevent erosion of the riverbanks and part of the riverbed, which is caused by the influence of waves and currents (GeoHooks BV, 2013; Witteveen+Bos).

Figure 13: Ground Consolidator element and the use of this concept on the riverbanks. Source: Witteveen+Bos, 2014

Vegetation development In erosive circumstances vegetation has no time to properly develop. Temporary protection in the form of biodegradable GC’s can form a solution to this by lowering the flow velocity near the surface and providing stable riverbanks. Therefore vegetation gets the opportunity to establish and grow. In addition GC’s provide protection against grazing by birds. By its temporary nature, this solution will provide a start for natural banks to develop as the biodegradable GC’s will be completely degraded after a few years (depending on material composition). After this, vegetation takes over the protection function of the banks. To enable riverbank protection by reeds the possibility should be given for the plants to grow for at least three years under favourable conditions with low water flow velocities. The use of GC’s in this way will also provide fish a shelter and a safe habitat where they can forage (Witteveen+Bos).

Biodegradable GC’s The biodegradable version of the GC’s are made out of a composite consisting of poly lactic acid which are strengthened with natural fibres like hemp or flax and when needed lime is added to provide more weight to the construction. The exact decay of the GC’s is not known since the degradation is a process done by microorganisms. The effectiveness of these microorganisms depends on surrounding environmental conditions and therefore the degradation process of the GC’s cannot be fully controlled. However the aim is to have 10 years of protection, which will gradually decrease. The rates at which the GC’s degrade also depend on the exact composition of materials used (Anome Projects, 2014). Cradle-to-cradle (C2C) is a concept where materials can be re-used in a new product without decreasing quality, which is often the case in the regularly used recycling approach. The C2C approach uses the rule: ‘waste is nutrition’ which means that all the residual products 23

should be re-used or should do no harm to the environment (Cradle to cradle, 2014). As described above the degradable version of the GC’s consist of polymers that are based on poly lactic acid, which is made out of corn. When the material is composting, this will provide nutrition for plants to grow which then again can be used for new polymers. Also the non- degradable GC’s, which can be removed after their efficiency has done its job in protecting the banks, can be re-used. The material can even be used to make new GC’s from it, which closes the loop again. The concept of the cradle-to-cradle cycle of non-degradable GC’s had been approved already after a three-year research in the harbour of Rotterdam where the GC’s were used as slope protection. And due to the stability of the construction, it is possible to reclaim all the individual GC’s and therefore no material is lost in the cycle. The final proof of the degradable GC’s being C2C is not there yet. A reed-fixing project and tests in Burgers’ Zoo are taking place in order to prove that also the temporary erosion protection mattresses of GC’s are C2C (Anome Projects, 2014).

Costs As described earlier the B-shore approach is interesting from both an ecological point of view, and also from an economic point of view, as the costs are not high in comparison to other solutions. Both low production costs and no maintenance- or disposal costs make this solution attractive compared to the more conventional solutions (Witteveen+Bos).

Pilot studies The concept was first tested in laboratories using scale models. This research proved that the GC’s were indeed reducing the waves. The wave reduction was depending on the amount of GC’s used and on the water height above the top of the construction (Boon van der, 2009). A pilot study was performed for one year in the Yangtze harbour in Rotterdam where a test section of 20x25 m was used. In this case first the bank was covered by geotextile with mats of braided willow branches on top of it. This mat construction was used to make sure the GC’s would be held in place. After this research it was concluded that the strength of the GC’s decreased over the year, but that there was no damage on the construction itself. The distribution of the GC’s remained the same, however there was compaction. Furthermore, it appeared that sedimentation was taking place at the slightly sloping parts of the bank but not at the steep parts. Unfortunately besides sand, also debris was trapped in the construction. But overall it was still concluded that the GC’s worked as a solution to protect the banks (Bouw & Teunissen, 2011; Anome Projects BV, 2011). According to Tom Wilms of the company Witteveen+Bos (Appendix A-g) the use of GC’s in the Spui could be a good opportunity with a lot of potential. The GC’s can also be combined with a layer of geo-textile beneath the cluster of GC’s. As described in the report of the pilot study in the Yangtze harbour in Rotterdam, first a layer of geo-textile was installed with on top of that a framework construction of bundles/mats of braided willow branches. After this one GC per square meter was secured to the framework in order to create a grid, which would provide a stable situation and connection in the end between the GC’s and the geo-textile. In order to uniformly distribute the GC’s per square meter 45 GC’s were spread by hand. Only after this preparatory work the mat is drawn to its final position and fixed using steel slags (Bouw & Teunissen, 2011). Although the research on the GC approach has been going on for five years, the concept is still relatively new and therefore the longer-term results of it are not fully known yet. Despite this, the pilot studies that are performed show positive results in the protection of banks against erosion and the development of natural vegetation. Therefore the potential for using this concept in the Spui is large.

4.1.1.3 Geotextiles Geotextiles are natural (biodegradable, for example jute or coconut) or synthetic (plastics) materials used to cover the riverbank. The geotextiles hold the soil in place while providing a rooting base for vegetation growth for further stabilization of the soil (Environment Agency, 2010; SEPA, 2008). Therefore it protects the soil from erosion. The (un)woven geotextiles are 24

pinned into place (see Figure 14) The biodegradable geotextiles have a shorter lifetime than the plastic geotextiles and are designed to control the erosion, until the vegetation takes over (Vlaamse milieumaatschappij, deel 1). Therefore, biodegradable geotextiles can enhance ecological value and biodiversity, also because the residues are decomposed and nutrients become available for the replaced vegetation (Tribute, 2013). The biodegradable geotextiles are also a hospitable environment for amphibians, insects or birds. The geotextiles can be used in combination with re-vegetation schemes, ground consolidators (GC’s), geotextile bags filled with sand and seeds or other initiatives to stimulate rapid vegetation growth (SEPA, 2008). A pilot study was performed for one year in the Yangtze harbour in Rotterdam with using geotextiles in combination with GC’s (see ground consolidators; Anome Projects BV, 2011). The use of geotextiles is not a new initiative, but the biodegradable geotextiles are less widely implemented which is mainly due to the lack of references and promotion (de Bruin, 2013). In case of the Spui, the use of geotextiles could be encouraged since it enhances biodiversity and reduces erosion. However, since the riverbanks are vegetated to a certain extent with grass and reed on top of stones, and other parts of the Spui are dominated with stones the use of geotextile could be questioned. This is because the stones have to be removed before implementing geotextiles, which could be very costly.

Figure 14: The biodegradable geotextiles are pinned into place. Source: Geo- synthetics, 2009

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4.1.1.4 Vetiver System For riverbank stabilization specific grasses, which decrease soil erosion on slopes, can be used. The application of the Vetiver System is one of the most prominent measures applying grasses, as it is used in more than 100 countries worldwide (Truong, 2000). In this system Vetiver grass (Chrysopogon zizanioides) is planted parallel to or across the water flow on the riverbanks in the form of narrow self-sustaining hedgerows (see Figure 15). The advantages of this grass species are that their deep, rigorous root system structurally stabilizes slopes against high flow velocity, while the shoots reduce surface run-off and trap sediments and seeds. Thus, growth of native species and increasing biodiversity in between the planted Vetiver grass rows is facilitated (Truong et al., 2008). Vetiver grass can be used widely as it is tolerant to extreme climatic variations like persisting drought, flooding, heat waves and frost. Furthermore, it tolerates wide ranges of soil characteristics, for example, soil salinity and acidity (Truong, 2000). It is especially useful for poor and highly dispersible and erodible soils. Additionally, it grows on very steep slopes (over 56°), is long lasting and low in implementation and maintenance costs. A planned maintenance program is only required in the first two years, but after establishing it is almost maintenance-free. Disadvantages of the application of Vetiver grass are for example the intolerance to shading. Hence, planting can only be done in open areas and weed control might be necessary during the phase of establishment. But this shading intolerance can also be desirable if the plant is just used as a pioneer for initial stabilization and improvement of microenvironments and hence enables later hosting of native endemic species. Furthermore, effectiveness of the Vetiver System is only given when the plants are well established (duration 2-3 month in warm weather, 4-6 months in cooler weather) and closed hedgerows are formed (Truong et al., 2008). Based on the given information, the Vetiver System would fit quite well to solve the riverbank erosion problem of the Spui. However, due to some discussion points we did not include it in our top five solution options. There exist for example no projects in Europe using the Vetiver System and thus evidence is missing that the Vetiver grass really is able to cope with the climate and soils in Europe and specifically in our project area. Still, regarding the given thresholds it should fit. Furthermore, the establishment phase of the Vetiver System could be too long with up to 6 months and no research results could be found on how effective other local plants establish between the thick root system of Vetiver grass. Thus in comparison to the given conditions now, biodiversity could increase or decrease. This option would additionally just tackle the erosion problem at the riverbanks but not at the riverbed in the Spui because the Vetiver grass is planted out of the water on the banks.

Figure 15: Vetiver grass roots minimizing erosion risks. Source: Holanda & Da Rocha, 2011, Chapter 5.1

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4.1.1.5 Planting willows Willows are called ‘bio-engineers’ in Tangelder et al. (2013), since these trees can be used to stabilize riverbanks and reduce the impact of waves. The willows roots reduce erosion because they act as a physical barrier and bind soil particles (Environment Agency, 2010). Willows planted as green stakes are useful, according to Water Policy Team (2010), in the reduction of erosion, but also in the increase of biodiversity by providing food for wetland species. The reduction of erosion by planting willows can be up to 90%, if properly managed. Planting willows is for example part of the ‘Building with nature’ project and has been applied in multiple areas in the Netherlands. For instance, in Noordwaard in Werkendam willows were planted to protect the area from inundation by reducing the impact of waves by approximately 80%, so increasing the height of dikes was unnecessary (IMARES Wageningen UR, 2013). The willows tolerate a flow velocity up to 10- 15 m/s (Vlaamse milieumaatschappij, deel 1). Rijkswaterstaat mentions in ECRR (2008) the importance of the degree of vegetation growth, since too much vegetation and trees can inhibit fast drainage at high peak discharges. This can reduce the flood protection in the area and therefore reduce safety. Also the distance of trees to the riverbank has an effect on the stabilization of soil. The project in Noordwaard was perceived successful, since the inhabitants did not experience much of a change in their environment and the problem could be solved through ecological functions. The planted willows might increase the landscape value, however competition for light can negatively influence other species (Vlaamse milieumaatschappij, deel 1). To decrease this competition for light and to optimize the stabilizing effect of willows they need to be pruned regularly. Planting willows are relatively cheaper than other hard measures. However, different types of willows require certain environmental conditions, like different durations and heights within the period of inundation (Vlaamse milieumaatschappij, deel 2), and can only survive in sweet water ecosystems (Tangelder et al, 2013). Therefore, it could be a constraint to plant willows at the riverbanks of the Spui, when opening the Haringvliet makes the area more brackish than the current situation (see open Haringvliet). Salix viminalis (Katwilg) is most tolerant to wet conditions, so for the Spui most appropriate. The willows in combination with stakes or woven willow branches have a lifetime of a couple years, (Vlaamse milieumaatschappij, deel 2), until vegetation can take over. Planting willows usually have a longer lifetime than just a few years. Although planting willows might be beneficial for the area of the Spui we did not select this option for the analysis, since other solutions might achieve the objectives of this research to a greater extent. It should be noted that planting willows can be very useful in combination with other protection measures.

Figure 16: Willow Mattress. Source: Salix, 2014

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4.1.1.6 Clay and sludge Elements already present in the riverbed of the Spui, or other rivers in The Netherlands, are clay and sludge. Mixing clay and sludge could create a strong natural soil that can be put on top of the current soil surface in order to prevent soil erosion (Van Den Brink, 2014). A mayor advantage is that clay and sludge are already there, so less transport costs and time. Even better, in some places in The Netherlands they have to get rid of the clay and sludge what makes this measure even lower in costs and therefore more attractive. The sludge has to be dried first and this process may take a while. It is for this reason that this measure needs some time before the clay and sludge can protect the riverbanks. Some actions are done to accelerate this process. The sludge has to be dragged out of the river itself first before stable structures can be formed, what can be expensive as well. Nowadays mixes of clay and sludge are used for dike protection in the Netherlands and some experiments to prevent bank erosion are going on in the - and in the surroundings of the Ijsselmeer. Clay with a higher amount of small particles has better structure, but also tend to fall dry very fast (Waterloopkundig laboratorium, 1985). Therefore, it is difficult to indicate the best type of protection measure. Adding sand will increase the rooting depth for plants, but makes the sticky structure between clay and sludge less strong. It looks like for this measure a choice has to be made between a strong erosion-proof soil (more clay) or more biodiversity (more sand). In this report we want to create both. For this reason we will not include this measure as a possibility for the Spui.

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4.1.2 Hard engineering techniques Hard engineering techniques embrace erosion protection solutions applying hard materials like stones, concrete or metal on riverbanks (Water Policy Team, 2001). Here we include using stone riprap, vegetated gabions, bio-cement, clay and sludge and ecological top layers.

4.1.2.1 Stone riprap Stone riprap or riprap revetments are terms which describe broken rock material that is placed either from the bank toe to a certain water surface elevation or along the complete bank height, usually parallel to the river flow (see Figure 17). Commonly used rock types are for instance granite, basalt and limestone (U.S. Department of the Interior Bureau of Reclamation, 2013; BC Ministry of Environments, Lands & Parks, 2000). The final rock slope should have at least the ratio 1:2 (vertical : horizontal). Thus, one meter in height should be horizontally extended by two meters of rock (Kosiw et al., 2008). In general, stone riprap reduces erosion by letting waves “roll-up” the slope instead of crashing into it. Therefore, the appropriate rock size is needed to hinder rock relocation caused by water movements from waves, currents or level fluctuation. To prevent erosion of underlying sediments and thus keep the rocks in place filter cloth, fill (grouting) and stone bedding layer are useful. Also planting of deep-rooting vegetation above or behind the rocks will accelerate the life span of the revetment. Next to erosion protection also stabilization of riverbanks is achieved with riprap (Kosiw et al., 2008). Further advantages of stone riprap are that it is already in use for a long time, it has a high durability (50 years) and flexibility, it does not fail under minimal shifting and is smoothly constructed and repaired (BC Ministry of Environments, Lands & Parks, 2000; U.S. Department of the Interior Bureau of Reclamation, 2013). Maintenance is usually low as the area only needs to be monitored and just if necessary, precautions or other measures then need to be taken (Broekhuizen & Van Spijk, 2014). To enhance floral and faunal habitat in this hard protection measure, textured rocks as well as gaps creating slow flow velocities should be used (Wilms & Te Slaa, 2014). But when constructing gaps, attention should be paid for only using locations with a low risk of underlying sediment erosion. Disadvantages of riprap are that it cannot be used on steep slopes as the rocks could encroach considerably into the river and thus cause opposite bank erosion. Also local scour and river deepening can be induced by riprap when easier erodible material below is relocated. Additionally, the installation of stone riprap may damage riparian or aquatic habitats. On the outside of a river bend the fish cover is limited due to the steep bank angle and higher flow velocities than in natural rivers. Also establishment of new depositional zones for vegetation growth is prevented. Thus, biodiversity of flora and fauna decreases and results in costs for mitigation or compensation measures. This does not apply to the interstitial spaces or “low- energy pockets” along the bank. Furthermore, construction time of riprap may be limited to a narrow period during fisheries window (BC Ministry of Environments, Lands & Parks, 2000; U.S. Department of the Interior Bureau of Reclamation, 2013). This erosion protection measure is the one already present at the Spui riverbanks. We do not want to keep it as this hard engineering technique has several disadvantages (see text before), especially the decrease of biodiversity and nature development carry weight.

Figure 17: Profile of stone riprap Source: N.C. Division of Coastal Management, 2014 29

4.1.2.2 Vegetated gabions Vegetated gabions are seen as soft bio-structural engineering techniques, which means hard structures are combined with soft measures (Environment Agency, 2010). Vegetated gabions can be implemented in the form of mattresses and baskets (see Figure 18). The baskets and mattresses are rectangular, made out of twisted or welded-wire mesh and filled with rocks. Gabion mattresses are flatter. These pervious and flexible structures are then stacked along the riverbank to reinforce steep banks (baskets) or they are placed directly and continuously on prepared banks to stabilize and protect lower banks and the bed of a river against erosion (mattresses) (McCullah & Gray, 2005). The advantages of the gabion on its own are the easy transport, application, successful decrease of erosion and relatively low costs for implementation and maintenance (Kosiw et al., 2008). Additionally, they are flexible and permeable building blocks (IDNR, 2006b). Due to supplemental usage of woody vegetation, which is inserted all the way through the basket to penetrate the native subsoil, habitat loss is at least slightly mitigated. Furthermore, the woody vegetation additionally reinforces the structure and increases longevity (McCullah & Gray, 2005). A disadvantage of gabions is that the durability is questioned when exposed to weather conditions, strong flows and large drainage areas. Usually it is recommended to use this method only for intermittent flows and small drainage areas. Another problem associated with gabions is to find the appropriate rocks and how to overcome the lacking provision of fish habitat (Kosiw et al., 2008). Although it is tried to prevent total habitat loss by using woody vegetation, nature development is almost completely eliminated. Gabions hinder sediment supply from local bank erosion and hence the establishment of new depositional zones where riparian vegetation could evolve (U.S. Department of the Interior Bureau of Reclamation, 2013). Looking at the given advantages and disadvantages of this solution option, the vegetated gabions are not an option for our project as admittedly erosion is decreased, but due to the hard protection measure of using the gabions, biodiversity and ecosystem services are almost reduced to zero.

Figure 18: Vegetated gabions: left side vegetated gabion baskets, right side vegetated gabion mattresses. Source: McCullah & Gray, 2005, p. 31, 36

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4.1.2.3 Bio-cement A rather new technique that can be used for the protection of the riverbanks is bio-cement (Van Den Brink, 2014), also known as soil-cement. For this measure cement, water and soil are combined to create a new kind of soil (Integrated publishing, n.d.). The cement will make the banks partly consolidated and form a strong and sticky structure with the natural soil. Best results can be obtained in clayey soils, since clay particle bonds are stronger than those of silt and sand; therefore clay soils can be quite resistant to erosion (PatsPage, 2014). In the first few meters of the soil near the Spui a lot of clay is present (Dinoloket, 2014). When used in an appropriate way, the erosion process near the banks will decrease. In contrast to the use of stone riprap on riverbanks, the mixture of cement with natural soil could still allow roots to penetrate. This is an advantage for using bio-cement as the biodiversity will increase. However, until what depth the rooting takes place might be dependent on the amount of the cement between the clay layers. A disadvantage of bio-cement is that it is a new technique in the Netherlands and therefore some pilot studies have to be done first. This is necessary to determine whether or not it is stable enough for the Spui and if it can increase biodiversity. For this reason we decided not to focus on this measure for the questionnaire.

4.1.2.4 Ecological top layer As stated before nowadays, stone riprap is found in a large part of the area around rivers. These blocks are important for the protection of riverbanks as well as for the safety of dikes. In case these blocks cannot be removed due to their stabilizing function, the question rises if there is a solution where stone riprap is used but where at the same time space for nature development can be created. De Bart & Meerburg (2014) explain that this can be achieved by using a natural system on top of the blocks: an ecological top layer. The idea is that part of the soil will be extended towards the shore, which means mosses and lichens are creating habitats on top of the stone riprap (see Figure 19). The method can be used for a very long time scale when the first pioneer mosses have survived. A very large disadvantage is that the stone riprap is still there and therefore a lot of increase in biodiversity would not occur at the transition between water and soil. The technique is rather a bit hard to maintain into the water, since the soil and pioneer mosses can flush away and the plain blocks are visible again. Therefore we think this measure is not appropriate for the Spui area.

Figure 19: Ecological top layer on stone riprap. Source: ArchDaily, 2008

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4.1.3 Other techniques The “other techniques” address the underlying causes of erosion, which usually need a lot of effort to be changed or stopped. Here we include the sand engine, widening the river, blocking the river and opening the Haringvliet.

4.1.3.1 Open Haringvliet For thousands of years the Dutch coast has been exposed to several major storms of which a few ended up in a disaster. The most recent one happened in 1953 in the southwestern part of The Netherlands and thereafter The Delta commission started large projects to defend the land and villages close to the rivers and sea in lowland areas. One of the results of the project is a dam with seventeen sluices between the Haringvliet and the North Sea, which came into force since 1970 (Deltawerken, 2004): the Haringvliet sluices (Figure 20). The dam reduces the risk of flooding during stormy weather. Moreover, it has a full time function to prevent salt-water intrusion in the Haringvliet and further land inwards. The seventeen sluices can all open in periods of high river discharge to avoid flooding and transport the fresh water into the sea (Deltawerken, 2004). Flow of seawater land inwards is not possible and therefore a hard gradient between fresh and salt water is created. In the scenario of an open Haringvliet this gradient will be gradual again which has advantages and disadvantages for different sectors in the area surrounding of the Spui (WNF, 2012). Figure 20: Upper: Location of the Haringvliet sluices and the Spui. Lower: The dam consistest of Salt water 17 sluices and only open when the river discharge is rather high. Source: Rijkswaterstaat, Due to climate change sea level rises, which causes 2008 the difference in water level between Haringvliet near the sluices and the North Sea to decline. When the sea level increases up to three meters compared to current values, no natural flow through the sluices is possible anymore (Hofland & Chamuleau, 2009). Opening the Haringvliet completely will in this case be a solution, even though it might be hard to except for people since of the changes that would that would occur described below. With an open flow towards the sea, salt water is able to penetrate more land inwards. Nowadays this is only the case at low river discharges and high water in the Nieuwe Waterweg. To prevent that salt water reaches more land inward places, the fresh water for the Haringvliet flows through the Spui in times of low tide. In case of an open Haringvliet, part of the water in the river will be salt and the Spui could not be used for preventing salinity anymore. Water from the Rijnkanaal in the north of The Netherlands is needed to make sure the brackish water will not reach the city of Gouda (Braakhekke et al., 2012). The entrance to the sea should be closed when there is not enough river water flowing through the Haringvliet to avoid salt intrusion. Instead of protecting the water quality, closing the dam for a couple of days has its counter sides. When salt water is trapped there is a chance that it will sink while the fresh water will remain above this layer. With a minimum of 30 days of stable and warm summer weather the river might get out of oxygen, resulting in mortality of organisms (MER Beheer Haringvlietsluizen, 1998).

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In addition, indirect impacts of salt water on humans have to be taken into account. The fresh water in this area is important for economic reasons (Braakhekke et al., 2012) such as irrigation water for agricultural purposes, drinking water and recreation. If for example the brackish water reaches the Biesbosch it would change the ecology as well as the drinking water supply for the city of Rotterdam.

Effect on protection of land in Spui area When the Haringvliet sluices will be opened the land behind will be in contact with the sea again. This has consequences for villages and agricultural land near the Spui, which are protected by dikes as the upper illustration of Figure 21 shows. In case of high water, ‘HW’ in Figure, stress of capacity increases and dangerous situations such as flooding could occur. The climate dikes (Rijkswaterstaat, 2009) as illustrated in the lower part of Figure 21 can be the solution for human Figure 21: Dikes near the Spui. Upper illustration for current safety. What makes them special is situation and lower illustration for a possible future their extra-long size pointing land construction. Source: Van Winden et al., 2010, p. 22 inwards what makes them stable (Vellinga, 2008). The climate dikes are very expensive once they are only used for safety reasons and not for other sectors in the Spui and along the Haringvliet (Hartog et al., 2009). However, the dikes are wide enough for agricultural land, recreation and housing and therefore make it become more suitable and cost efficient. The lower picture shows that for creating climate dikes one usually needs to widen the river. In case of the Spui this might not be necessary since the volume of water the Spui transports would decline since water from the Haringvliet can directly be transported towards the sea. The water level difference between the Nieuwe Waterweg and the Haringvliet would decrease, what makes the pulling force of the Haringvliet disappear. Consequence that also for this process the water volume in the Spui declines. For the climate dikes along the Haringvliet widening of the river cannot be excluded, because there would probably be no reduce in water volume. Furthermore the dikes should be prepared for climate change taken into account the expected peaky discharge of the river Rhine. During stormy weather the sluices can close occasionally, but one has to keep in mind the lack of oxygen described above. It is important to note that with all sluices opened the Haringvliet can

Figure 22: Sedimentation increase by opening the Haringvliet would stop the erosion and increase biodiversity by sandy soils. Source: Braakhekke et al., 2012, p. 22.

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transport 6000 cubic meters water towards the sea. When the whole dam is gone, this would be 3 times more (Broekhuizen & Van Spijk, 2014). Broekhuizen & Van Spijk (2014) state that only in a completely restored Haringvliet the erosion in the Spui will be solved, but in this case we cannot use the dam as a storm surge anymore when needed. Another point of discussion with respect to the effects of the open Haringvliet is the decrease of the velocity and the protection measures. The current velocities in the Spui are rather high due to the tidal movements from the sea by the Nieuwe Waterweg (Mosselman, 2014). With an open Haringvliet this tidal influence would decrease and therefore the speed of the water declines. One could suggest that less riprap is needed, because the decreased velocity will reduces the erosion, but on the other hand the dikes are than exposed to the open sea water and more protection might be needed for safety reasons (WWF, 2010). Sediments from the sea and/or Haringvliet are transported more land inwards and create a more stable shore in case of an open Haringvliet. Figure 22 illustrates, in Dutch, that a partly opened Haringvliet would already partly solve the erosion problem in the Spui due to the increase of sedimentation (Braakhekke et al., 2012). With a completely opened Haringvliet, even sandy soils might be found and therefore other species of flora might develop than what currently exists in the clayey soils. One has to keep in mind that the largely increased discharge with an open Haringvliet, could also reduce sedimentation (Broekhuizen & Van Spijk, 2014) and then there will not be sandy soils.

Ecological benefit in Spui Due to the salt intrusion in the Haringvliet the ecology would change into other similar estuarial waters. Examples are the return of the salmon and salt tolerant plants (MER Beheer Haringvlietsluizen, 1998). In the MER report (1998) other positive ecological benefits are discussed, such as the population of birds. For the Spui it is important to know whether or not the brackish water would reach the border Middelharnis – Spui and move further into the Spui. Rijkswaterstaat (2011) wants to close the sluices again when the salt water reaches the Spui. If Rijkswaterstaat does not change this decision, salt tolerant plants would only occur in the western part of the Haringvliet, but not in the Spui. Besides the possible negative effect of the salt water on biodiversity, the scenario of an open Haringvliet can have positive effects as well. Namely, when bringing back the tidal influence of the North Sea in the Haringvliet, natural dynamics could be reintroduced resulting is a reduce of riverbed erosion (MER Beheer Haringvlietsluizen, 1998). The lower speed of the flow in the Spui could create a more suitable environment for animals that live in the water and along the riverbanks.

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4.1.3.2 Blocking the Spui River In nota 16-1953, Rijkswaterstaat mentions for the first time the blocking of the Spui in respect to the programme that is then implemented (Valken, 1953). The plan consisted of two dams on both sides of the river to block it completely. The plan was primarily focussed on flood safety , however it would also have other positive side effects, such as stopping salt-water intrusion from the sea during drier periods. The cost-benefit analysis of Rijkswaterstaat showed a positive outcome (Valken, 1953). The expected costs were estimated to be 7 million guilders for the first year, while the benefits for the subsequent years would be 1,6 million guilders. The project would break-even only after 4,5 year, which is a relatively short time frame. The blocking of the Spui can be realised by placing dams on either side of the river or to close one side of the branch (Valken, 1953; Vermeij & Neefjes 2014). Because of these dams, there is no estuarial dynamic in the area anymore, making erosion not a problem anymore (Litjens, 2014; Broekhuizen & Van Spijk, 2014). Blocking the river at both sides would, according to Rijkswaterstaat (Valken, 1953; Vermeij & Neefjes 2014), be preferable to the situation of blocking only the Oude Maas side. Blocking the Spui at the Haringvliet side would have only negative consequences for salt-intrusion, navigation and safety. Blocking at two sides has the advantages that there is protection against storm surges, it can be used as a fresh water reservoir, there is the possibility to reclaim land from dike areas and road traffic has more opportunities to cross the river (ibid.). The disadvantage would be that the has more water to handle, which means that the problem could be relocated, and shipping in the Spui would be impossible or would be delayed because of sluices. In addition, the natural tidal dynamics, which are currently in the area, would be abolished; this has impact on the current biodiversity in the area. In order to maintain the current level of water quality, it is required that there is a flow in the river. For this, it is essential to provide a fresh water inlet of water, so that the Spui can be flushed when necessary.

Biodiversity impact An advantage of a closed area is that the surroundings of the river can then be given back to nature to develop since lower dikes are possible in this area, this might increase biodiversity to a certain extent. On the other hand, biodiversity could decline because of this. The natural system that is currently in the Spui area would be altered; the tidal dynamics of the area would not be in place any more. This would mean that the area would face the same consequences as the Haringvliet, in which, after the enclosure, the ecological systems are entirely changed. The estuarial dynamics, now present in the area, make a very special ecological zone, which, with damming, would be abolished. The biodiversity that can emerge in the area thereafter would solely be based on fresh water, since the Spui would change in an enclosed lake.

Socio-economic impacts Enclosing the area and the resulting lowering of the dikes would result in possibilities for recreation alongside the rivers. The current velocity of the water is strong. By closing the water body, the river could be used for recreational purposes as swimming, canoeing and other water related activities. Damming would, on the other hand, make through shipping impossible. Through shipping is now almost all of recreational use. Another option is to include sluices, which would make navigation from and to the Spui possible, but this would delay shipping. In addition, this option would be more expensive than just damming the river, since a sluice complex has to be built. Another impact for the area lies with the impact on fresh water. The fresh water supply of the area must also be changed; this would need a pipeline from the Biesbosch to accommodate for intrusion of salt water (Deltaprogramma, 2012).

Safety Closing the Spui can also be used to protect the area from high water (Vermeij & Neefjes, 2013). Flood risks would be prevented in the Spui area, since there is no direct relation to the sea anymore; only if one of the dams would break, flood risks play a role. A big disadvantage is that it would only solve the problems in the Spui area; the erosion problems would relocate, since the 35

Haringvliet will pull water towards itself. The flow of water in Dordtsche Kil and Oude Maas would therefore be expected to increase, with negative consequences on these rivers such as increased erosion and higher flood risks in those areas (ibid.). The costs of mitigating these problems would probably be high. This impact is however questioned. In a report by Rijkswaterstaat (Valken, 1953) it is mentioned that the flows in the Dordtsche Kil would probably only change with a small percentage. This is in the current situation, with the Delta works in place. It could, however, be a choice that to promote biodiversity in the Spui area, the other two rivers can be covered in concrete. The natural development will then only occur in the Spui area.

Salt-water intrusion During droughts, the fresh water pressure of water flowing to the sea is declining. Therefore, during high tides, it is possible for salt water to intrude into the estuary and even intrude in the Spui area. During, for instance, the drought of 2003 the Spui turned into a brackish river, preventing the use of the fresh water inlet points Bernisse and Beerenplaat. Beerenplaat is a collecting and storage basin for fresh water. Evides purifies this water to make it suitable for consumption. Evides applies a maximum acceptable chloride concentration of 150 mg/l. Bernisse is an inlet for the northern side of the Spui, distributing water to the canals around the Harbour of Rotterdam (Litjens, 2014). The inlet of Bernisse can guarantee a fresh water supply of 23.000 l/s to its hinterland for water level management, salt flushing and irrigation. It is expected that because of climate change drought periods will be longer and more frequent and consequently result in more salt intrusion in the Spui area. Closing the Spui would also prevent backward salt-intrusion until the Haringvliet (Vermeij & Neefjes, 2013). The basic assumption is that the Spui closure results in a larger seaward flowing river volume via the Oude Maas and the to the Nieuwe Waterweg (Vermeij & Neefjes, 2013). Therefore a reduced effect onto the chloride concentration along the Hollandse IJssel is predicted. It is assumed that the salinity reduction of this measure is larger for locations along the Nieuwe Maas and Nieuwe Waterweg (ibid.).

Impact of climate change on water flow in the Rhine As can be seen in Figure 4 the IPCC expects the Rhine discharges to change. More about this can be found in the Chapter 1.3 on climate change impacts. The extremes are expected to increase; this means longer periods of as well droughts as high water levels. The river is able to carry discharges up to a level of 50,000 cubic meters per second. Only in case of the highest estimated discharge, this number is exceeded, which means that the riverbanks are expected to be safe. Drought periods, on the other hand, cause more problems. As said before, with low discharges, the salt seawater intrudes the area. In this A2 scenario it is expected that low discharge events will increase significantly, thus a higher potential exists for salt-water to intrude in the Spui area. Blocking the Spui area would prevent this.

Temporal closing Temporarily closing the Spui with a flood defence is, by some, considered as a high-opportunity measure (Zethof, 2011). Vermeij & Neefjes propose in the Gebiedsrapportage IJsselmonde an inflatable rubber dam (Balgstuw). This will also cause enough pressure on the salt-water, since water will not flow into the Spui and can be used to pressurize the intrusive salt water. The advantage of a temporal closing is that the salt water-intrusion is currently not very common, so that a temporal closing would be enough. It is estimated that currently the salt-water intrusion will only be around one or two tidal cycles (Zethof, 2011). The fresh water reserves in the Spui are then sufficient to overcome the closure. Because of this, it is not necessary to complete an expensive water pipe to the Bernisse and Beerenplaat inlets (Deltaprogramma, 2012; see Figure below).

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Figure 23: Map of the area with Bernisse and Brielse Meer. Source: Google Maps

However, as shown, the consequences of climate change will have an impact on the river Rhine, resulting in longer drought periods, which means that the river must be blocked for longer periods. The question is if, when blocking the Spui at the Oude Maas side, there will be enough fresh water available for these two inlet points. The pulling forces of the Haringvliet now make sure that fresh water is running through the Spui out of the direction of Oude Maas. If this side is blocked, fresh water cannot longer flow into the river from this side and only from the Haringvliet side. However it stops salt-water intrusion from the Nieuwe Waterweg during droughts (since it is blocked), the fresh water supply of the Bernisse and Beerenplaat may be in jeopardy. In addition, a temporal closure does not stop erosion during the periods that the flood barriers are closed. The only advantage of this solution seems thus that salt water cannot longer intrude into the Spui during longer drought periods. Consequently, closing one side of the Spui does not seem a good solution for this project. However, since it is both positive for preventing erosion and on the other hand promotes biodiversity (although it is a different kind of biodiversity), this option can be considered as fruitful for the development of the Spui.

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4.1.3.3 Widening the River The option for widening the Spui is inspired on the Dutch Room for the River program from 2007 (Planologische Kernbeslissing Ruimte voor de Rivier, 2007). The purpose of this program is to prevent flooding of the major river systems Rhine, Meuse, and IJssel. The plans aims to improve the discharge to the sea of the rivers during high discharge events. If possible, room for nature development can be created as well, however this is not a main goal of the project. The history of the plan can be found in Plan Ooievaar of 1986, which was made to improve the spatial quality of the river basins (Ruimte voor de Rivier, 2014). Another event that inspired the project was the extreme water levels of the large river systems in 1993 and 1995. The near flooding in these years caused an understanding of urgency at the Ministry of Transport, Public Works and Water Management (V&W) and Rijkswaterstaat (Ministerie van Verkeer en Waterstaat, 2000). It was then assumed that large-scale measures were necessary. The Room for the River project consists of forty different measures to improve the discharge improvement of the river. Examples include floodplains and removing obstacles in the river, see the Figure below for more examples. The focus on river expansion was new for the Netherlands. Before the Room for the River project, dike improvement was the major method of improving flood defences in order to encounter the enlarged flood risk (Helmer, et al., 2004). The project is complicated since it includes many different actors. Besides the already mentioned Ministry of V&W (now the Ministry of Infrastructure and the Environment; I&M) and Rijkswaterstaat, the Ministry of Economic Affaires, Agriculture and Innovation (ELI), multiple water boards, provinces and municipalities need to be involved (Room for the River, 2014). The first measures were already taken in 2006 and the project runs until 2015. The total costs are estimated on 2.3 billion euros (ibid.).

Figure 24: Measures of Room for the River (Dutch). Source: Factsheet Ruimte voor de Rivier, 2007

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Application to the Spui In accordance with the Room for the River program, the river will get more room to flow. Because the water volume is flowing through a larger area, the water velocity will drop, thus preventing erosion and maybe even promoting sedimentation (Factsheet Ruimte voor de Rivier, 2007). For this solution to be useful the river must be widened two or even three times the current size (Litjens, 2014). However, more research needs to be done on discharge levels. One stakeholder of Stroming claims that because of the pulling forces of the Haringvliet, erosion could become even larger. This is because even more water could then flow into the Haringvliet via the Spui. However, other experts claim that by giving more room to the river, the width and the depth of the Spui are more in relation to each other. In addition, this situation would be more natural, since the embankments of the Spui have subsequently been moved in the direction of the water thus resulting in an increasing narrower river.

Impacts on biodiversity The current problem of the Spui is that the ratio between the width and the depth of the river is not in relation to each other. This is because the severe erosion had deepened the river. The shores of the river are therefore not very natural. With this solution, the shores can be made more natural, with a gradual transition from land to water, so that nature can develop around the shores. Besides this, additional agricultural land that will be claimed for the widening the river will be transformed to river systems. A positive outcome of this changing land-use is that there could be more room reserved for ecological purposes than in the original situation. If extra room for nature is given, it is necessary to claim even more land from farmers, which makes the project more expensive.

Socio-economic impacts The widening of the river will, as been said before, have impact on the land use in the area, for instance, on land that currently has an agricultural purpose or land that is used for housing or fresh water cleaning. This means that these functions have to be moved to other locations. A positive outcome of this changing land use is that more space could be reserved for recreational purposes; this will improve popular support. An increase in popular support is necessary, because many projects that reclaim agricultural land for rivers are contested. An example is the de-poldering of the Hedwigepolder, although according to the MER (Milieu-Effect Rapportage, 2013) the environment will benefit from the de-poldering, the popular support is very low in the area, especially round farmers. In the Spui area, this can be seen as well. One of the farmers in the area did not want to move to another place and started a lawsuit (Litjens, 2014). This land was necessary to enlarge an ecological zone near the Spui River. In the end, he won the case, which meant that he did not have to move. The adjacent land-pieces surrounding his land are currently transformed into nature. One of the advantages of the clustering of houses around the villages in the area is that this solution around the Spui would probably not mean that many people have to move. This also increases popular support and this diminishes costs. Figure 25 shows the area that can be used to enlarge the river with minimal consequences for housing. The yellow line shows a doubling of the river, while the red line shows a tripling. If done correctly, the esthetical value of the area could greatly increase resulting in a new nature area.

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Figure 25: Map of the Spui with lines indicated which ground would be used with doubling (yellow) and tripling (red) of the river. Source: Google Maps, own adaptations

Costs The largest disadvantage of this solution is that it is relatively expensive. Dikes must be removed, the river must be enlarged and new dikes must be created. The land that would be necessary for this is already in use, which would make it costly to purchase. The dispossession of land is a complicated procedure and is arranged in the dispossession law of 1851. The government tries to arrange the dispossession in an organized way by first discuss with and consult the person that will be dispossessed. This complicated matter would cost significant amounts of time and money. The construction of the river is complex as well. From examples in the recent history, it can be said that large-scale projects tend to be out of budget. As this is a complicated case, this risk could also be present with the widening of the river Spui. On the other hand, enlarging the river would cut on maintenance costs. Disposal of blocks might not be necessary anymore since there is a more natural flow if erosion is indeed stopped.

Long-term vision As stated in the part on climate change, discharges are expected to be lower in the dry seasons in the summer. Salt-water can then intrude in the delta. With the widening of the river, this problem is not taken care of. Probably this plan is therefore not climate proof. More research is necessary on flow velocities and the intrusion of salt-water in this case.

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4.1.3.4 The sand engine The sand engine (or motor) is an experiment in the management of a dynamic coastline that was put in use in The Netherlands for the first time in the world. At the request of the Hoogheemraadschap van Delfland, as part of the coastal management and the maintenance of the coastline by the central government and the province of , a peninsula was created between Ter Heijde and Kijkduin, where natural beaches and dunes are relatively narrow. The sand engine is considered an influential building with nature project. The artificial peninsula of about two square kilometres was created in 2011 (Stive et al., 2013). The idea is that the currents, waves and wind deposit the sand in a natural way along the coast, thus protecting the coast. Usually, extra sand is deposited on the beaches every 5 years (Mulder & Tonnon, 2011). However, it is expected that with the sand engine, this is not necessary for the coming two decades. This method is expected to be more cost effective and also helps nature by reducing the repeated disruption caused by dredging and replenishment (De Zandmotor, 2014). In general, the political and public perception of the Sand Engine has been positive over the last 18 months and the first results suggest that the engine is efficient in re-enforcing the beaches (Stive et al., 2013). For our project, a sand engine is probably not a useful option. We thought that it could be useful since it is a new, innovative way of coastal enforcement. Another aspect why it could be useful was the system of preventing erosion and promoting sedimentation. However, the situation in the Spui is different than the coastal situation and there are two points why it probably would not be applicable for the Spui River. Firstly, it is not applicable because there is only research done at open sea, and not at smaller water bodies. The location of a sand engine in this project would be in the Haringvliet. A second point is that it probably will not improve the ecological quality of the area. The deposition of sand will not occur in the Spui area. The existing currents are too strong to allow sedimentation; consequently, this solution would neither solve erosion nor ecological problems. With the opening of the Haringvliet, this plan can however be interesting; the currents in the Spui will drop and this will promote the chance on a successful implementation of the sand engine. Also, for a natural distribution of sand, this plan can be of importance if the Haringvliet will be opened.

Figure 26: The Sand engine. Source: Stive et al. (2013)

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4.1.4 Overview (dis-)advantages of potential erosion protection measures

As there are many potential erosion protection measures introduced in the texts before, the following table gives an overview of the measures and their advantages and disadvantages (see Table 4).

Table 4: Overview of advantages and disadvantages of all potential erosion protection measures; Grey background highlights the top five measures

Erosion protection Advantages Disadvantages measure Soft engineering techniques Willow mattresses in - Reduction of flow velocity - Toe protection is probably necessary combination with toe - Trapping of sediment - Mattresses should be well anchored protection - Provision of habitats - Large amounts of material required - Immediate measure - Possible consequences due to disease or - Rapid vegetation growth insect infestation Ground Consolidators - Prevention of riverbed and bank erosion - No other (secondary) functions possible due to wave reduction - Debris is trapped - Biodegradable - Relatively new concept, thus not all - Vegetation gets chance to grow, thus impacts are explicitly known increased biodiversity - Low installation/no maintenance costs Geotextiles - Stabilize riverbanks and decrease erosion - Costly to remove stones in the Spui to - Increase biodiversity (e.g. amphibians) prepare soil for geotextiles - Enhance ecological value (biodegradable - Biodegradable geotextiles shorter life time geotextiles)- could be used in combination than plastic geotextiles with GC’s Vetiver System - Tolerant to extreme climatic variations - Depending on location establishment such as persisting flooding, heat waves and phase 2-6 months frost - Shade intolerant (can also be advantage - Tolerant to wide ranges of soil when Vetiver grass used as pioneer) characteristics like soil salinity and acidity - System is only effective when closed - Especially useful for poor and highly hedgerows are formed and plants are well dispersible and erodible soils established - Grows on steep slopes (steeper than 56°) - Long-lasting - Low in implementation and maintenance costs - Resistant to high velocity streams Planting willows - Stabilization of riverbanks, decrease of - Short lifetime wave impacts and erosion - willows grow only in sweet water systems - Tolerant to flow velocities up to 10-15 m/s - Regular maintenance needed (e.g. - Increased biodiversity (wetland species) if pruning) properly managed - Light competition can reduce non-willow - Previous willow projects successful species - Increased landscape value Clay and sludge - Effective stabilization of riverbanks - Decrease of biodiversity in comparison to - Increase of biodiversity in comparison to soft engineering techniques solely using cement - Dragging sludge out of the river is expensive Hard engineering techniques Stone riprap - Preventing erosion - On steep slopes rocks could encroach into - Stabilizing banks the river and cause opposite bank erosion - Used for a long time (already a lot of - Local scour and river deepening experience with this measure) - Installation of stone riprap may damage - Easily constructed and repaired, riparian or aquatic habitat maintenance unnecessary - Decrease of biodiversity of flora and fauna - High flexibility and durability/long life resulting in costs for mitigation or span (50 years) compensation - Prevention of new depositional zone establishment Vegetated Gabions - Effective in reinforcement and - Durability is questioned under the stabilization of steep riverbanks (baskets); elements, strong flows and in large drainage

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stabilization and erosion protection of areas lower banks and riverbeds (mattresses) - Problematic to find the appropriate rock - Easy transport and application for each specific side - Relatively low costs for implementation - No provision of fish habitat and maintenance - Additionally added woody vegetation mitigates habitat loss, supplementary reinforces structure and increases longevity Bio-cement - Effective stabilization of riverbanks - Decrease of biodiversity in comparison to - Increase of biodiversity in comparison to soft engineering techniques solely using cement Ecological top layer - used for long time scale - not much increase of biodiversity due to - not many costs, since stone riprap is stone riprap already there - might not be used under water Other techniques Opening Haringvliet - Salt water creates biodiversity - Large area and lot of stakeholders are - Reduces erosion on a natural way involved - New dikes could be expensive Blocking the river - Dikes that are not necessary anymore can - End of the estuary dynamics in the river. be removed so that additional space to - Relocation of problems to other areas nature can be given. - Probably the cheapest option to prevent erosion in the area. Widening the river - Possible to do without loss of many houses - Relatively expensive - More space for environment and ecology - Complex to construct - Esthetical value of solution is large if done - Farmer land will be used for room for the correctly river - Safes on maintenance costs - Popular support is not present for these kind of solutions (see Hedwigepolder) - Salt intrusion? Sand engine - New, innovative idea - Not enough research done for river implementation - Seems not a correct measure for the Spui area.

In the questionnaire, it was asked if one was familiar with a certain solution. In the graph below, the results can be seen (Figure 27). This figure shows that the Open Haringvliet is the solution that is best known, followed by widening the river and willow mattresses. Ground consolidators and blocking the Spui are less know solutions.

Are you familiar with this solution?

Willow Matrasses

Ground Consolidators

Blocking the Spui No Yes Open Haringvliet

Widening the River

0 5 10 15 20 25 30

Figure 27: Question in questionnaire: Are you familiar with this solution?

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4.2 Questionnaire Below you can find the results of the direct ranking done by the stakeholders and the arguments that were stated by the stakeholders in the open questions in the questionnaire. In the direct ranking the stakeholders were asked to rank the five solutions that were provided to them and as a sixth option they could choose also to keep the situation as it is now, the status quo. In the open questions the stakeholders were asked to give their opinion on the provided erosion protection measure and to provide arguments with that. The results of these open questions are given per solution per stakeholder group in section 3.4.2.

4.2.1 Preferable solutions by stakeholders In the Figures below it can be seen that most of the stakeholders gave the preference to the Open Haringvliet solution, followed by Widening of the river at the second place. The third ranked option, is the solution of using Willow mattresses and after that the other soft engineering technique the Ground Consolidators. Keeping the current situation as it is was ranked fifth, which means that, Blocking the Spui was indicated by the stakeholders as worst solution. Especially in this last case, there is a rather general opinion about this solution ranking it as worst solution, which was done by 17 out of the 30 stakeholders. Also for the solution with an Open Haringvliet there is not a large distribution between the positions that the stakeholders chose for this solution. For the other four options the distribution of the ranking is larger. There was, in general, an agreement amongst most stakeholders which solution is best and which worst. The respondents from the agriculture group were the ones that least agreed with the other groups (see Table 5). In this table, 1 stands for a highly preferred solution and 6 for a not preferred solution. Widening the River and Opening the Haringvliet score in general good, with both an average of 2,7. However, the agriculture group, ranked these two options, together with blocking the Spui, worst. This is probably because the land used for widening the river is agricultural land. For the open Haringvliet, farmers fear the salt-intrusion in the area. This direct ranking provides, together with the arguments provided by the stakeholders, information to compare to the indirect ranking, which is done in the Multi-Criteria Analysis.

18 16 14 12 Best 10 Second Third 8 Fourth 6 Fifth 4 Worst 2 0 Widening the Open Blocking the Ground Willow Keep current River Haringvliet Spui Consolidators Matrasses situation

Figure 28: Self-ranking of options by stakeholders from questionnaire

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Table 5: Self-ranking of options by stakeholders from questionnaire. 1 is high preference, 6 is low preference

Keep the Widening Open Blocking Ground Willow current the River Haringvliet the Spui Consolidators Mattresses situation n

Consultancy/Advice 2,5 2,8 4,9 3,8 3,8 4,5 8

Agriculture 5,5 6,0 6,0 2,0 2,5 4,0 2 Engineering 3,5 4,0 4,0 4,0 3,0 3,5 2

Inhabitant/volunteer in the area 2,7 2,3 5,6 4,1 3,5 4,4 10

Nature organisation 2,0 1,8 5,0 3,8 4,0 5,5 5 Government 2,0 2,0 5,0 3,7 3,7 5,7 3 Total 2,7 2,7 5,2 3,7 3,6 4,6 30

6

5 Consultancy/Advice

4 Agriculture

3 Engineering

2 Inhabitant/volunteer in the area

1 Nature organisation

Government

Figure 29: Self-ranking of options by stakeholders from questionnaire. 1 is high preference, 6 is low preference

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4.2.2 Arguments provided for solution options

4.2.2.1 Widening the river

Consultancy/Advice Six out of the eight stakeholders in this group are in agreement that this solution will lead to nature development, but on how recreation possibilities will evolve there is some disagreement. Two stakeholders mention that recreation possibilities can increase, another says it should not be the primary focus point and again another is uncertain about how it will evolve. By bringing back more natural dynamics safety is argued to increase by two of the stakeholders, but then it should not just be given more room in the floodplains, also the carrying capacity of the river itself should be increased a lot, and not just creating more room in the floodplains. There is one stakeholder that states that the erosion will only increase when the river is widened as the tidal forces induced by the Haringvliet will remain the same. Widening will therefore lead to more water flowing through the Spui. Only when the river is made wide enough that the proportion of water volume corresponds to the width of the river, the erosion will decrease.

Agriculture Only one of the stakeholders in this group responded to this solution by mentioning that it is better to tackle the consequences of the erosion rather than the cause since this will be cheaper and it will cost less space.

Inhabitant/Volunteer According to this group the main advantage of this solution is that it will lead to a more natural system restoring the original tidal dynamics with space for nature development. In addition, two of the stakeholders mention that erosion will decrease and that therefore also costs will decrease in the end. However, five of the respondents argue that it will need a large area to implement this solution and therefore agricultural land needs to be purchased with accompanying high costs and resistance of farmers.

Engineering The two stakeholders in this group mention that it is an attractive solution for the landscape scenery, but that a large area is needed, which will involve high costs, and that it is a complex procedure.

Nature Organisation All respondents mention that a large area is needed for this solution, but that it will lead to a more natural river system. There, opportunity is given for nature development due to more natural transitions of land surface to water.

Government Four out of the six stakeholders mention the high costs involved in this solution. They also mention the option is too radical and not realistic due to the sacrifices that need to be taken and the uncertainty of its effectiveness. The other two stakeholders argue that it will create opportunity for nature development and recreation, but one of them also mentions the possible decrease in water quality.

Overall, it can be seen that the government group is hesitant towards the solution. The groups Inhabitant/Volunteer, Engineering and Nature Organisation mention that a large area is needed which is not what they want. Furthermore, all the stakeholder groups, except for the agriculture group, mention nature development and restoration of the natural dynamics of the river as an advantage.

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4.2.2.2 Opening Haringvliet

Consultancy/Advice Five of the stakeholders in this group mention that there should be paid attention to the salt water intrusion in this solution, since it will have consequences for the locations of water inlets and it will involve high costs when they need to be relocated. Four of the stakeholders argue that restoring tidal dynamics in the area by the introduction of this solution will have beneficial consequences for nature development and therefore the ecological value increases. In addition, it is argued by two respondents that fish migration will be enhanced.

Agriculture Only one of the stakeholders in this group responded to this solution by mentioning that with opening the Haringvliet a large fresh water basin will disappear for which there is not a good alternative.

Inhabitant/Volunteer Again the more natural tidal dynamics and the need for relocation of the water inlets are mentioned by two stakeholders. However, there is another respondent who argues that in the past before the Haringvliet was closed, it was also possible to let fresh water in at the stations in the area. Therefore, he argues that the current stations can probably still work for a long time and in the meanwhile the water supply in the whole delta can be adjusted. Furthermore, there are two stakeholders in this group who firmly believe that there are no disadvantages of this solution, while two other stakeholders are more hesitant and first want to see how a small opening of the Haringvliet will work out.

Engineering Only one of the stakeholders in this group responded to this solution by providing the argument that salt-water intrusion will become a problem.

Nature Organisation All respondents agree that this solution will lead to a more natural tidal river system. One of them also mentions that the salt-water intrusion might cause difficulties in the fresh water supply for agricultural and drinking water purposes.

Government By restoring the natural tidal dynamics in the area, flow velocities will be lower, which is beneficial for the erosion problem solving. This is argued by two stakeholders in the group. However, according to four stakeholders the solution will have high risks caused by the salt- water intrusion. This will negatively impact the water supply for agricultural purposes and also the fresh water inlet at Bernisse, which in its turn will be disadvantageous for the Brielse Meer.

In conclusion, it is again the government group, which shows most doubts about the solution. However, they also mention, just as three of the other stakeholder groups, which this solution will lead to a more natural tidal river system, which has ecological benefits for the area. The other two groups, agriculture and engineering, do not provide this argument. All the groups, except for agriculture, do show their concern regarding the salt-water intrusion, which can cause problems for the fresh water supply in the whole area.

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4.2.2.3 Blocking Spui

Consultancy/Advice This solution can lead to another type of ecosystem with more opportunities for recreation and it is probably a cheap solution for the problem in the Spui. This is argued by two stakeholders in this group. However, these arguments are the only advantages that are mentioned. Four respondents argue that this solution will only relocate the erosion problem to other rivers in the surrounding area. In addition, the removal of tidal dynamics can cause the water quality to deteriorate and there should be taken care of the water quality in the inlet at Bernisse which might become salty. When the blockade is situated east of the inlet it might be an option according to one of the stakeholders. However, if the blockade is located west of the inlet you will have to deal with an accumulation of brackish water coming in the Spui via the Oude Maas according to another respondent.

Agriculture Only one of the stakeholders in this group responded to this solution by mentioning that it will only relocate the erosion problem, but that it might solve the problem in the Spui.

Inhabitant/Volunteer Half of the stakeholders in this group mention the issue of relocating the problem to surrounding rivers such as the Dordtsche Kil, the Noord and also the Oude Maas, which will then experience an increase in the erosion problem as the pulling forces by the Haringvliet, will not change. Another mentions that natural dynamics in the river will be lost completely. This is confirmed by two others, who mention that the water quality will probably deteriorate with algae blooming and exchange of organisms as main issues.

Engineering Only one of the stakeholders in this group responded to this solution by mentioning that it is a possible solution, but that it might give difficulties in navigation.

Nature Organisation The stakeholders mention different arguments for this solution. One of them mentions the issue of relocating the problem, which will require hard measures to be taken in the surrounding rivers. This is disadvantageous for ecology and natural dynamics there. Another respondent argues that it will give more opportunity for nature development and that it will reduce the safety risk for flooding.

Government Only the possible stimulation for recreation is mentioned as an advantage of this solution, which is done by two stakeholders in this group. Furthermore, relocation of the problem is also mentioned by two respondents. Three of the stakeholders argue that this solution will have negative impacts for the fresh water inlet at Bernisse. In addition, it is expected by three respondents that there will be no tidal dynamics anymore, which is disadvantageous for biodiversity in the area.

Overall, the main drawback given in the feedback for this solution is that it will only relocate the problem to rivers in the surrounding area. In addition, the lack of tidal dynamics causing water quality to deteriorate is also mentioned within three of the groups: Consultancy/Advice, Inhabitant/Volunteer and Government. Nature development as well as biodiversity were not mentioned a lot within all of the groups but by arguing that tidal dynamics will disappear and deterioration of water quality, it is indirectly referred to.

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4.2.2.4 Ground Consolidators (GCs)

Consultancy/Advice Three stakeholders in this group mention that the use of GCs will not prevent the erosion problem of the riverbed. One of the respondents even argues that the erosion will only increase when the banks are fixed using GC´s thereby decreasing the current resistance, which causes a larger pulling force from the Haringvliet. There is also some doubt about the sustainability and effectiveness of this solution. Two stakeholders think the flow is too high and the transparency of the water is too low for vegetation to develop. But when they have the chance to establish, it would enhance nature development and it would give rise for more habitats to develop. It is perceived as a cost efficient solution by two respondents. However, the solution will not provide secondary functions in the area besides bank protection.

Agriculture By the two respondents in this group it is perceived as a good solution, which is worth to further investigate.

Inhabitant/Volunteer In this group several arguments are given for the solution, but there is not a general view on it. Two stakeholders mention that the flow velocity will be too high for vegetation to develop and also the current lack of transparency of the water is mentioned as a restriction for vegetation growth. Therefore, its effectiveness is doubted, but it might be good to combine this solution with other measures as is suggested by one respondent. When the vegetation has the opportunity to establish, the ecological value will increase. However, stabilizing the banks will not solve the major erosion problems of the riverbed and the estuarine dynamics in the area will not be restored.

Engineering In this group the two stakeholders mention that they doubt the effectiveness of the solution and that they expect the maintenance costs to be high.

Nature Organisation Stakeholders give different arguments for the solution. Argued by one of the respondents is that the estuarine dynamics will not be restored using GC’s and the other respondent is uncertain about the sustainability and the chance of success regarding the current high flow velocities in the Spui.

Government The solution is perceived rather positive in this group. Some just mention they think it is a very good solution and that further research should be done. Another respondent think this solution will enhance biodiversity. But there are some questions raised too: for instance about the effectiveness of the solution due to the high flow velocities and that it might be good to combine this solution with other measures to increase the effectiveness in solving the problems in the area. Another respondent expects the flow velocities in the middle of the Spui to increase, which is disadvantageous for the riverbed erosion rate.

In conclusion, uncertainties regarding the effectiveness of the solution are raised, by all the groups except for the Agriculture group, considering the current high flow velocities in the Spui. In addition, the erosion problem the riverbed is facing, will not be solved using this solution according to stakeholders within the Consultancy/Advice, Inhabitant and Government groups. Furthermore, on the one hand, the estuarine dynamics in the area will not be restored using GC’s, but on the other hand it might increase the number of habitats available, thereby stimulating biodiversity.

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4.2.2.5 Willow mattresses

Consultancy/Advice The same argument given for the GC’s applies for this solution as well, according to three stakeholders within this group. It refers to the erosion of the riverbed, which - just as using the GC’s - will not be solved using willow mattresses, and one of the respondents argues again that it can even lead to more erosion in the riverbed caused by the pulling forces of the Haringvliet. But it will stabilize the riverbanks using natural material and it can provide new habitats, which will enhance the ecological value, which is argued by two respondents. It is argued that it might be a good solution when combined with other measures, since on its own it will not be a sustainable solution as argued by two stakeholders. They mention that it does not solve the cause of the problems and it will not make the delta climate proof.

Agriculture By the two respondents in this group it is perceived as a nice solution, which is worth to further investigate.

Inhabitant/Volunteer Several arguments are given for the solution in this group. There is no general view on it within the group. On the one hand two stakeholders mention that there are advantages for the area from an ecological point of view and that it will create habitats, which are attractive for birds and other animals due to the use of natural materials. On the other hand it is argued that there are too high flow velocities in the Spui for the mattresses to establish well and that the mattresses should be combined with other measures to also solve the erosion problems of the riverbed. Four stakeholders think that the solution looks nice and promising.

Engineering The two stakeholders mention that the effectiveness and longevity are not clear to them. But one of the stakeholders argues that it is a more natural solution for bank protection than the use of GC’s.

Nature Organisation Only one of the stakeholders in this group responded to this solution by mentioning that just as with the GC’s it is uncertain how sustainable the solution is regarding the current situation with high flow velocities.

Government One of the stakeholders compares it with stone riprap and argues that the mattresses will be a sustainable solution too, while another stakeholder in the group raises the question whether it is sustainable regarding its life span. However, two respondents think that it would be good to combine the mattresses with other measures. It is also argued by one stakeholder that this solution will not reduce flow velocity much and therefore will not solve the erosion problem. Furthermore, one stakeholder thinks that the mattresses could help to improve biodiversity.

Overall, when stakeholders compare this solution to the other bank protection measure Ground Consolidators, the willow mattresses are perceived as a more natural solution. But again often the argument is given that the mattresses are also not able to solve the erosion problems of the riverbed and its effectiveness and sustainability are questioned. However, according to stakeholders within three of the groups, Consultancy/Advice, Inhabitants/Volunteers and Government, this solution will create more habitats, which in the end will enhance biodiversity.

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4.3 Multi-criteria analysis and sensitivity analysis Here the results of the MCA are given. They are divided in the three steps, which need to be conducted to get a final result from the MCA: standardization, weighing and ranking. Subsequently, a Sensitivity Analysis was done to evaluate the robustness of the ranked erosion protection measures. 4.3.1 Standardization/Scoring The standardization or scoring of criteria in the MCA was done on the basis of literature research and expert interviews, for example about erosion protection measures and the Spui area. Hence, knowledge on positive and negative performance of measures on criteria could be transferred in scores from –- to ++. Figure 30 below gives an overview of the scoring end result. Zero thereby means there is no impact, not positive or negative, on the Spui area if the new erosion protection measure is implemented. By using the sub criterion “Navigation”, an example for scoring will be shown. From the investigations it is known that the measures Ground Consolidators, willow mattresses, opening the Haringvliet and keeping the current situation will not impact on navigation (0). For the measure of widening the river more space is given for ships passing each other. But widening the river could also mean that the water level is lowered, thus just one plus is scored. When blocking the river no shipping is possible anymore to get from one river through the Spui to another river. Hence, a minus is scored. More explanations on why all other scores were given can be found in the Appendix C-I.

Figure 30: Screenshot of scoring table in the MCA (BOSDA 3.1). Left: group- and sub criteria; Upper right: top five erosion protection measures in comparison to keeping the current situation; ++ is most positive, 0 means no impact, -- is most negative 51

4.3.2 Weighing The following Figure 31 shows the weights stakeholder groups gave to the five main criteria “Safety”, “Costs”, Ecological value”, “Socio-economic criteria” and “Diverse land use”. The criterion “Safety” received from three stakeholder groups (Agriculture, Engineering, Inhabitant/Volunteer) the highest weight of all criteria (ranging between 37.5-70.0%), often combined with high weights for “Costs” or “Ecological value”. The other three stakeholder groups (Consultancy/Advice, Nature Organisation, Government) assigned most weight to the criterion “Ecological value” (ranging between 42.5-47.5%), combined with high weights for the criterion “Safety” or “Costs”. Criteria with lowest weights given per stakeholder group differed a lot, except that “Safety” never obtained the lowest weight. In general, the criterion “Costs” never received more weight than 25%, “Socio-economic criteria” and “Diverse land use” received maximum 15%.

Figure 26: Pie charts of weighed main criteria (numbers given in %) determined by six different stakeholder groups

4.3.3 Ranking To keep it simple, the results of the Multi-Criteria Analysis (MCA) will be mainly explained solely based on the ranking and without focusing too much on the contribution of different criteria weights to that ranking. Thus, detailed bar charts can be found in Appendix C-II, but are just sparsely discussed. Resulting from the MCA, table 6shows the ranking of the top five erosion protection measures for the river Spui in comparison to the rank of keeping the current situation of the river. Three of the six stakeholder groups (Consultancy/Advice, Inhabitant/Volunteer, Nature Organisation) ranked according to their given weights first “Widening the river”, second “Open 52

Haringvliet” and third “Blocking the river”. The remaining three groups (Agriculture, Engineering, Government) differed in that order, but kept the same measures and ranked first “Blocking the river”, second “Widening the river” and third “Open Haringvliet”. Reason for these three erosion protection measures to be always ranked among the first three is their performance on the group criterion safety. Safety contributes most to the fact that the three measures have considerably higher total scores in comparison to the remaining three measures and thus high ranks (see Appendix C-II). For the fourth and fifth rank almost all stakeholder groups had the same results, except the Agriculture and Engineering group (see Table 6 below). Thus, “Willow mattresses” was mostly ranked fourth and “Ground consolidators” fifth, or in the case of the Agriculture and Engineering group both, “Willow mattresses” and “Ground Consolidators”, on the fourth rank. “Keeping the current situation” was always ranked lowest in comparison to all the other five options of changing the erosion protection measure. This is due to the low performance on costs (see Appendix C-II)

Table 6: Ranking of top five erosion protection measures as a result of the Multi-Criteria Analysis, compared to the rank of keeping the current situation

Erosion protection Widening Open Blocking Willow Ground Keep measures the river Haringvliet the river mattresses consolidat current ors situation Stakeholder groups Consultancy/Advice 1 2 3 4 5 6 Agriculture 2 3 1 4 4 5 Engineering 2 3 1 4 4 5 Inhabitant/Volunteer 1 2 3 4 5 6 Nature Organisation 1 2 3 4 5 6 Government 2 3 1 4 5 6

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4.3.4 Sensitivity Analysis The table below shows the outcome of the Sensitivity Analysis conducted on basis of the results (ranking) of the Multi-Criteria Analysis (MCA). Zero means that there is no distance to the rank of the best alternative (erosion protection measure). Hence, an alternative with zero distance is actually the best alternative. Empty cells (-) imply that there is no weight combination where an alternative would be ranked first. Numbers (possible maximum is 1.0) in light grey cells give the distance to get on the first rank. The higher the number, the larger is the distance. From this distance it is possible to infer to the sensitivity of the first rank because an alternative with a high number is less sensitive to changes in weights. Thus, changing to the best rank is more difficult and therefore a certain robustness of the first rank exists. The left table column gives names of stakeholder groups and additionally the distance of the best-ranked alternative to change to another lower rank. The first rank (widening the river) of group Inhabitant/Volunteer is most sensitive to changes in weights as the distance to leave the first rank is the lowest value of all (0.06). Instead, the open Haringvliet would be ranked best because it has the lowest distance amongst the alternatives to change to the first rank (0.08). The first rank of the Government group (blocking the river) is less sensitive to changes (0.12) than the one of the group Inhabitant/Volunteer. Widening the river would be ranked best if slight changes are done to the weights given by stakeholders. Further lower sensitivity shows the first rank (widening the river) of the group Consultancy/Advice (0.28). Changes in weights would result for the Consultancy/Advice group in ranking first to block the river. The other first ranks of the remaining three stakeholder groups (Agriculture, Engineering, Nature Organisation) are even less sensitive (higher distance number) and hence are seen as kind of robust.

Table 7: Results of Sensitivity Analysis showing distance of an alternative (erosion protection measure) to be ranked first; zero = first rank; numbers in cells of stakeholder groups = distance of first ranked alternative to change to a lower rank; light grey = distance of alternative to be ranked first; dark grey = never ranked first;

Erosion protection measures Widening Open Blocking Willow Ground Keep (alternatives) the river Haringvliet the river mattresses consoli- current Stakeholder groups dators situation Consultancy/Advice 0.28 0 - 0.27 0.34 0.52 - Agriculture 0.42 - - 0 0.41 0.47 0.75 Engineering 0.36 - - 0 0.69 0.68 - Inhabitant/Volunteer 0.06 0 0.08 0.21 0.42 0.58 - Nature Organisation 0.33 0 - 0.35 0.63 - - Government 0.12 0.12 - 0 0.33 - -

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5 Discussion This section of the report discusses the outcomes of the research. First, the methods will be discussed. Thereafter, we look into the results of this study and finally, some general remarks are given. 5.1 Methods In this chapter, we will look into the methods of the research. The questionnaire will be discussed first, and secondly, the multi-criteria and sensitivy analysis will be covered.

5.1.1 Questionnaire The response rate of the questionnaire was good, out of the forty requests, circa two thirds filled out the questionnaire, which is relatively high. With more time, it would have been possible to sent the questionnaire to more people to perform a quantitative analysis so that the results would have been more scientifically proof. Another manner of improving the response rate is printing the questionnaire and interview citizens of the area and other stakeholders directly. The responses on the questionnaire were good as well; the respondents viewed it as not too long and thought it was an interesting research. For the analysis, we have used most questions, although not all questions seemed to be necessary for the report. This was however limited to only two questions that, afterwards, were not used so much. We should have mentioned in the questionnaire that people should fill in their names and if they want we could make them anonymous in the report. 5.1.2 Multi-criteria and sensitivity analysis We used to ways of Multi-Criteria Analysis to control ourselves. The first one was the use of the programme DEFINTE; this programme is a BOSDA-MCA. To control the results, a second, hand- made MCA was made in excel. The results of these two multi-criteria analyses was largely the same, therefore we have decided to only use the DEFINTE MCA in the report. The excel file can be obtained by the writers of this report. The extra control step was good, so that we did not automatically thought that the results were good to use in the report. The weak point of a MCA is that it can be altered. There are a few steps in the progress of creating a MCA that can be considered as more or less subjective. These steps are, the decision on which criteria to use, the weighing of the solutions and the weighing of the criteria. In this research, the weighing of criteria is done through questions in the questionnaire. By grouping and averaging these responses, a more general idea can be obtained. However, more respondents would have made this process more scientifically proof. The decision on which criteria to use and the weighing of the solutions is done by us. This is done through interviews with experts and literature research. Each expert gave us options for these criteria and these were sometimes controlled at interviews with other experts. This last part could have been done in a more structural manner; for instance, we could have asked the experts if they agreed on the criteria and the ranking of the solutions in the end. Unfortunately, there was not enough time for this. In addition, it might have been appropriate to be more open about the choices we have made in respect to the criteria and solutions. This would have improved the reliability of the research.

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5.2 Results This sections looks into the results of the research; the questionnaire will be analysed according to the five different options from the long-list. 5.2.1 Questionnaire Besides the direct ranking, which was done in the questionnaire sent to the stakeholders, people were also asked to give their opinion on the provided five solution options. They should also support their opinion using arguments in favour and against the solution. This was supposed to provide more clarity on how they weighed the criteria in the first part of the questionnaire. The total number of stakeholders invited by e-mail to fill in the questionnaire was 48 of which 30 stakeholders responded by sending the completed questionnaire back. This response rate is of 65% is quite high considering that most stakeholders were only approached via e-mail. Besides the e-mails that we have send directly to certain stakeholders, we also received an e-mail in which we could see that the questionnaire was forwarded to several other people. On one hand, this is beneficial to reach as many stakeholders as possible. On the other hand, we tried to control to whom specifically we would send the questionnaire according to our definition of a stakeholder. Some of the respondents gave a clear overview of pros and cons in order to underpin their opinion, while others just shortly mentioned that they think it is a good or a bad solution. The latter was also the case for one of the respondents in the groups Agriculture and Engineering and sometimes they did not fill in an answer at all. Since both of the groups are only consisting of two stakeholders, the opinion of the other stakeholder within these groups gets a relatively large weight. Therefore, it would have been better if we would have set as a requirement that each group should have a certain minimum number of stakeholders within them. Unfortunately, this was not possible due to the limited number of stakeholder responses. Below you can find the main arguments in favour or against a certain erosion protection measure, which were provided by all groups of stakeholders.

5.2.1.1 Widening the river The main arguments given by the respondents of the questionnaire for the solution widening the river are that a large area will be needed and that high costs are involved to realize the option. Due to the creation of more room for the river the area of agricultural lands along the Spui will decrease. But, when enough land is claimed this solution option will give more space for nature development and it might increase area with a recreational function. Most important is the question, whether besides increasing chances for nature also the erosion problem will be tackled and if at the same time safety can be guaranteed. Some respondents argue that by restoring the natural dynamics in the Spui safety will increase and in addition, if the carrying capacity is increased by widening the river it can store higher discharges, which will be beneficial in the prevention of flooding. There is some disagreement about the feasibility of the solution since a large area is needed which includes high costs. But most of all local inhabitants need to be informed about the project in order to get their support.

5.2.1.2 Opening of Haringvliet The main arguments given by the respondents for Opening of the Haringvliet is that natural tidal dynamics will be restored which is beneficial for nature development and for solving erosion problems in the river system, but that the intrusion of salt water further inland could give problems for the fresh water supply in the surrounding areas. Especially there are concerns regarding the supply of fresh water for the surrounding agricultural fields and for the inlets of drinking water. All respondents share the opinion that the water supply should be changed in one way or the other when the Haringvliet sluices are opened. There are respondents who also mention that the combination of opening of the Haringvliet together with widening the Spui

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could be interesting to do further research on. In general, this solution is received rather positively as it will restore the area towards a more natural estuary.

5.2.1.3 Blocking the Spui Almost all respondents agree that blocking the Spui will only lead to relocation of the erosion problem to surrounding rivers in the area. Further measures would be needed in those rivers. It is mentioned as a drastic solution by some of the respondents in which the tidal dynamics of the Spui will completely disappear. This is advantageous for solving the erosion problem in the Spui itself, but the effect on the biodiversity is uncertain and could be negative. Another point of concern that is raised by the stakeholders is that measures should be taken to guaranty the supply of enough fresh water at the inlet for Bernisse. Blocking the river would also have negative effects for navigation. Although it is not a main transport route, when blocked it cannot serve as an alternative route anymore when there are obstruction problems in the surrounding waterways. Despite these disadvantages some respondents think it is the cheapest solution of the given options. And most stakeholders think recreation in the area will be stimulated. But the general opinion is that this solution will relocate the erosion problems and by blocking the river the tidal dynamics of the estuary will disappear completely and this is not wanted by most stakeholders.

5.2.1.4 Ground Consolidators The use of Ground Consolidators should be a good solution to stabilize the riverbanks, but it will not solve the erosion problems of the riverbed, and it might even enhance it according to several stakeholders. And since the erosion is mainly an issue for the submerged parts of the river, the riverbed, this option will not solve the erosion problem in that part as vegetation will not grow there. Another point of concern that is raised by stakeholders is that the principle is not a natural solution and that estuarine dynamics will not be restored by using the Ground Consolidators. Therefore, it is only part of a bigger system of solution for the area according to some stakeholders. There are also concerns whether the flow velocity in the Spui is not too high for vegetation to establish and survive in these circumstances. Several respondents think it could be a good solution for the short term, although they mention that further research should be done to determine its effectiveness in solving the erosion problem and the development of nature. 5.2.1.5 Willow mattresses in combination with toe protection The willow mattresses will enhance the natural development and give opportunities for habitats to be created. Therefore, it will increase biodiversity and the ecological value, which is mentioned by five stakeholders. However, the flow velocities in the Spui might be too strong for the willow mattresses to be effective, which decreases the sustainability of this solution. Furthermore, the erosion problem of the riverbed is, just like with the Ground Consolidators, not solved by using the mattresses and it is mentioned that this way of fixing the banks will lead to more erosion of the riverbed, too. So it might be a good option to use the mattresses in combination with other measures to solve the erosion problem and to increase biodiversity. The solution by itself is not sustainable in the sense that it will not solve the cause of the erosion and therefore it will not make the delta climate proof according to a respondent.

The arguments that were given for the individual solutions within the stakeholder groups were diverse and not consistently pointing in one direction (see Results – Questionnaire). This can be either due to the grouping of the stakeholders done by us but which was based on the sector they work in indicated by themselves, or it can be due to the number of stakeholders per group that was not sufficient to register their main view upon a solution option. It would therefore have been better to have more stakeholders to fill in the questionnaire. But it should also be considered that not all the stakeholders are experts. This can cause that some of the individual stakeholders within a group have expertise on the solutions or the problem that is stated and 57

therefore can provide substantiated arguments, whereas other individuals within the same group could not have sufficient knowledge to provide solid arguments. Although the latter group might not have the expertise and therefore the reliability on the arguments they provide can be questioned, it is still important to include in the analysis as they are also the people that should provide support to the solution in the end. The most used arguments that were provided by the stakeholders are in agreement with information provided by the experts or that was found in literature. But some arguments were distinctively different (see Results – Questionnaire) from all the other arguments. These arguments were mainly provided by people that we could identify as experts with knowledge on the area and on the specific solution they provided an argument for. Unfortunately not all respondents of the questionnaire indicated their name or company they are working for. 5.2.2 Multi-Criteria and Sensitivity Analysis Standardization of criteria was done on the basis of knowledge gained from literature research and expert interviews. Therefore, it is assumed that all scores given within this step of the MCA are scientifically sound. Still subjectivity played probably a big role. Thus, a recommendation for a future standardization is to double-check the given scores by experts again. In general, it is difficult to discuss the pie chart distribution of weights per stakeholder group (see Chapter 4.3.2 Weighing) as this is a result of a grouping process. People being active in the same sector were grouped and their opinion (weights) on criteria was averaged. But despite having a job in the same sector, people have different backgrounds, interests and values. Also different numbers of people represent one group. Thus, when trying to find a reason why for example the Consultancy/Advice group gave even more weight to “Ecological value” than the Nature Organisation group, there is no general answer possible. As a result, a discussion on why certain stakeholder groups may have given which amount of weight is left out. Still it can be concluded from the pie charts that for most of the people within different groups “Ecological value”, “Safety” and “Costs” were the most important criteria. This shows that most people prefer an erosion protection solution for the Spui area with high ecological value that means increasing biodiversity, less salt intrusion and restoring natural dynamics such as sedimentation. Aiming for high safety implies for example that dikes or other protection measures are expected to still resist peak river discharges and thus protect people’s houses and fields. Also considerable weights are given to costs, what reveals that a certain balance between costs and benefits of the erosion protection measure is desired. Within the Multi-Criteria Analysis the ranking of alternatives resulted for all six stakeholder groups in either widening or blocking the river as the most preferred erosion protection measures. As it was a draw for both alternatives (3:3) no conclusion on the best alternative can be done so far. Thus, further investigations and discussion follow. The related Sensitivity Analysis showed that in some stakeholder groups (Advice/Consultancy, Inhabitant/Volunteer, Government) a certain sensitivity of the first rank to weight changes was present. But even with small changes widening or blocking the river dominated the best ranks. To stay within the restriction of the report, only these two alternatives and opening of Haringvliet (mostly ranked second or third) will be discussed further. Based on literature research and expert interviews, Chapter 4.1 Potential erosion protection measures already extensively discussed the advantages and disadvantages of the mentioned alternatives. Now the question is which solution stakeholders finally see as the best solution and which arguments they give. Therefore, the ranking outcome of the MCA will be compared to the answers of the questionnaire, where stakeholders needed to give a ranking on their own. A comparison is necessary because the first ranking in the MCA represents an indirect ranking of stakeholders. Here the given weights were just used as an input to the program and combined with scores from other people. Hence, the result might not be what stakeholders intended. The second ranking was directly given and no further processing was necessary to know the preference of stakeholders. Within the MCA there was three times first ranked blocking the river but stakeholders actually saw it in the direct ranking as the worst solution of all alternatives because (as already 58

mentioned earlier) the erosion problem will be relocated, tidal dynamics are removed and biodiversity is thus deteriorated. Reasons for still being on the first rank within the MCA is first that scores for this alternative were often quite positive in comparison to other solutions (see Chapter 4.3.1 Standardization/Scoring of criteria). Second, stakeholder groups Agriculture, Engineering & Government, which had as MCA outcome blocking the river on first rank, gave higher weights to the criterion “Costs” than other groups (see Chapter 4.3.2 Weighing). Blocking the river is generally scoring better in “Costs” than widening the river or opening the Haringvliet (see ranking bar charts in Appendix C-II), hence the result was the best rank for this alternative. Also widening the river was three times ranked first by stakeholder groups (Consultancy/Advice, Inhabitants/Volunteer, Nature Organisation) within the MCA, but in contrast to blocking the river, a lot of stakeholder this time agreed in the questionnaire that it is a good solution (in direct ranking nine times ranked first). Reasons they gave are - as already mentioned earlier - that widening the river will decrease erosion, give more space for nature development, restore natural dynamics and the carrying capacity and thus increase safety. For the MCA especially high weight on the criterion “Ecological value” in combination with low weight on “Costs” was crucial for the first rank because widening the river scores well for the former criterion but worse for the latter (see Chapter 4.3.1 and 4.3.2). Concluding from the comparison of direct and indirect stakeholder ranking it can be stated that the result from the MCA – draw 3:3 between widening and blocking the river - changed due to direct stakeholder ranking to a clear best rank of widening the river. Starting the whole approach (of what is the best rank when MCA and questionnaire ranking are combined) from direct ranking would imply that opening the Haringvliet is the best alternative. Half of all stakeholders rank it as best solution to the erosion problem and the decreasing biodiversity and nature development in the Spui area. But it needs to be kept in mind that the MCA is an outcome where experts and stakeholders gave their opinion. Expertise is very important to find the best solution for the area. Hence, the MCA ranking gives a more scientifically sound result than the direct stakeholder ranking. Therefore, widening the river is the best alternative. Considering the result, still some discussion exists whether or not this alternative really reduces erosion in the Spui area. Most experts agree that widening a river reduces flow velocity and thus erosion (Rohde, 2005). But others also state more water could flow through the river Spui due to the pulling tidal forces of the Haringvliet and thus even more erosion could happen (Litjens, 2014). To counteract this process, a combination of widening the river and opening the Haringvliet could be considered. As a result, the pulling tidal forces of the Haringvliet would largely decrease and flow velocities would actually be reduced (see Chapter 4.1.3.1 Open Haringvliet). From this example it can be seen that spatially only focussing on the Spui area is not sufficient as interdependencies with related water bodies and landscapes need to be considered as well. Therefore, further research on a larger spatial scale should be done in the future to find even more possibilities for achieving the project objective of decreasing the erosion problem and increasing biodiversity and nature development in the Spui area. At the same time potential impacts on adjacent landscapes and water bodies should be taken into account to not just relocate the problems.

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5.3 General remarks The results of the MCA can only be used as qualitative analysis, since 30 stakeholders filled in the questionnaire (see explanation in paragraph 3.4). It should be mentioned that different stakeholders gave arguments for or against certain solution options, but not all arguments existing were captured. Probably just the most important arguments or the ones they came up with in that moment were filled in. This is mainly because of their restriction in time and the large number of questions asked in the survey. In addition, not all stakeholders we intended to reach, filled in the questionnaire, like Evides or more stakeholders for a particular group. For instance, the intention was to contact more farmers through Land- and Tuinbouworganisatie (LTO, agriculture and horticulture organisation) and fisheries, but unfortunately they could not be included. It is important to note that the farmers and/or fisheries, who are mostly impacted by the changes in the Spui area, did not have a lot of influence in the results. Furthermore, in the Spui area the responsibilities for the project design, future project planning, maintenance, costs and other responsibilities are not often straightforward. This makes it difficult to distinguish who can be held accountable or asked for specific information on the area. In addition, the different kinds of expertise of the experts make the conclusions or arguments sometimes misleading. Since the discipline of their expertise plays the largest role in their decision which solution fits best. This is because without perhaps having an overview of the whole current situation and all possible consequences of a solution in the future for other disciplines as well. Moreover, during the interviews it became clear that some experts were stuck in old traditional structures and not willing to accept new measures. Only a few of them were willing to think out of the box or had investigated on different, new techniques. Of course it is not wrong to keep with the old measure and improve it, but this often meant that these experts were not willing to accept that other techniques were possible. This sometimes created miscommunication between experts, so some stated that one solution could be perfect for the Spui and the next expert said it would be impossible. The solutions itself have been reduced from a list of 14 solutions to a top 5 of best solutions. The most optimal situation would be a combination of several solutions, but in the scope of this research it was too time consuming to do so. This is due to the fact that it would take all combinations of a list of 14 solutions to analyse which combination would be the most effective (see Table 8). Also, it would be clearer to focus on one objective, for instance increasing biodiversity, instead of weighing objectives all at once. The outcome then often has implications for other objectives. Therefore, it could be better to combine measures, to reduce implications caused by one solution with another solution option. It is also questioned whether or not to use a solution for the whole river, and not specific parts of the river. This is not specifically taken into account in this report, but could be of importance when investigating the feasibility, opportunities and costs of a particular solution. Another point necessary to address is the differentiation of bed erosion versus riverbank erosion. These types of erosion are linked, as one affects the other as explained in Chapter 3.2. Not only the combination of solutions can be a point of discussion, but also the temporal scale of the solutions. When answering the question ‘what is the most effective solution’ it should be clear what is actually meant by this: the effect on the short term or long term. On the short term the solution is for instance an immediate decline of the erosion without considering future climate change scenarios. Or the solution could be only effective when the solution is based on these scenarios. Short-term solutions are also very useful when considering the uncertainties of climate change, so when the climate responses faster or slower than expected the measures can be adjusted. The discussion on the role of climate change in the design of a solution option should be adequately addressed. Also because the weighing of investments made on the long- term profit or short-term results can be difficult. The large uncertainty about the role of climate change also complicates the analysis of the most effective solution.

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Table 8: Methods associated with strategies

Strategies Promote Increase Manage Increase Effecting Improve Alignment Available Sediment Biodiversity Salt Natural Stability Area to Intrusion Development Methods the River Geotextiles X X Vetiver X X X X System Planting X X willows Willow X X X X mattresses Ground X X X X consolidators Stone riprap X Vegetated X Gabions Bio-cement X Clay and X sludge Ecological X X top layer The sand X X X engine Widening the X X X X river Blocking the X X X X Spui River Open X X X X Haringvliet

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6 Conclusion The focus of this study was to decrease the erosion problem, while enhancing biodiversity and nature development. Through literature study, interviews with experts, and a questionnaire filled in by stakeholders, the most effective solution was searched for in the Spui area. We found that the results of the Multi-Criteria Analysis (MCA) were different than the results from direct ranking (the open questions in the questionnaire). The ranking from the MCA showed a draw between the solution of widening the river and blocking the river, but we argue no conclusion can be drawn from this. Blocking the river was not favourable in the arguments given by stakeholders in the questionnaire. Thus, we infer widening the river is the best outcome from the analysis. In comparison to the arguments given in the open questions in the questionnaire, the solution of opening the Haringvliet turned out to be the most favourable solution. Half of all stakeholders rank this as best solution to the erosion problem and decreasing biodiversity and nature development in the Spui area. However, the MCA ranking gives a more scientifically sound result than the answers given directly by the stakeholders. Therefore, we conclude that widening the river is the best alternative. The results have shown that the groups Inhabitants/Volunteer, Consultancy/Advise and Nature organisations (the more ‘soft’- oriented groups) suggested the same solutions in the same order where widening the river had most potential. The groups Engineering, Agriculture and Government (the more ‘hard’- oriented groups) favoured the solution blocking the Spui. We also saw that keeping the Spui as it is scored very low for all groups. A striking outcome was that the inhabitants favoured widening the river compared to the government who preferred to block the Spui. In addition, it needs to be considered that a lot of different arguments (often contradicting) and opinions were given in the questionnaire, but also during the interviews. Even though widening the river came out as best solution for this research, the best way of reaching the objectives (to decrease erosion and increase biodiversity and nature development) would be to combine several solutions. Combinations could be for instance widening the river with an opened Haringvliet and using ground consolidators on riverbanks. When different solutions are analysed in order to find a best solution, a more sectoral approach is used (which is currently happening in most cases). However, an integrated solution option could better achieve the objectives by a combination of measures. So, we argue further research is needed in order to investigate the most optimal combination of measures. Additionally, testing this combination of measures to current climate change scenarios is important too. The time scale of the effectiveness of solutions needs to be taken into account, especially with trends like the lower summer peak discharges of the Rhine and salt-water intrusion. As mentioned earlier the spatial scale of the solution could have large impacts, for example the erosion problem could be solved in the Spui area but can be relocated elsewhere. The weighing of costs on the short and long term scale are also highly relevant. In order to find a most effective solution or combination of solutions local support and the involvement of stakeholders is very important. To make a concluding remark, it is difficult to capture an overview of all arguments, opinions and attitudes towards the ‘soft’, ‘hard’ or ‘other’ erosion protection measures. Where different stakeholders with different interests are involved it remains a challenge to find an optimal solution for all parties. Although this research can be seen as an exploratory research and needs further study, it has shown some important suggestions and considerations. Most importantly, it has shown that there are several opportunities in the Spui for both decreasing erosion and increasing biodiversity and nature development.

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Acknowledgements

We would like to express great appreciation to our commissioner Elma Duijndam for her support and feedback. Special thanks we also give to Gijs van Zonneveld and Bas Roels for their contribution and to Gerard Litjens who showed us around in the area of the Spui. Also we thank Claus van den Brink (Arcadis), Jan Kruijshoop (Rijkswaterstaat), Marjolein Meerburg and Hans de Bart (Waterschap Hollandse Delta), Arie Broekhuizen and Ary van Spijk (Rijkswaterstaat), Tom Wilms and Steven te Slaa (Witteveen+Bos) for the interviews held in the preparatory phase of the project. In addition, we thank the stakeholders who filled in the questionnaire. We thank Hasse Goossen and Saskia Werners for their guidance.

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Appendix

A. Interviews

a. Interview Schedule (Dutch)

Vragen

Sleutelwoorden: Schaal, efficiëntie korte en lange termijn, type oplossingen, multifunctioneel, erosie, innovatie, criteria

1. Welke maatregelen/oplossingen worden er genomen om erosie te verminderen of zelfs sedimentatie bevorderen? (stroomsnelheden). 2. Hoe ernstig is het erosieprobleem in uw ogen? 3. Zijn er andere adaptatie maatregelen om erosie tegen te gaan behalve het veranderen van de oever en bodembescherming? Meer ruimte voor de rivier? Meer of minder dynamiek in de rivier? 4. Zijn er eco-engineering beschermingsmaatregelen mogelijk i.p.v. stenen? Zo niet, hoe kun je het gebied aantrekkelijker maken qua biodiversiteit? 5. Worden de huidige maatregelen (hard measures) gemonitord? Is it necessary to look at flow velocities? 6. Weet u of er nieuwe innovatieve erosie beschermingsmaatregelen zijn bedacht die nog niet uitgevoerd zijn die interessant voor ons project kunnen zijn? Van welk soort/type maatregelen profiteert de natuur optimaal? 7. Hoeveel zullen erosiemaatregelen in de toekomst kosten? 8. Hoe geleidelijk moet de overgang zijn van de huidige maatregel naar een meer duurzame oplossing? 9. Denkt u dat het noodzakelijk is om de lokale bevolking erbij te betrekken of kunnen de plannen effectief en duurzaam worden vanaf de top- down approach? 10. Op welke schaal moet dit gebeuren om efficiënt te zijn? Is er een verschil in kwetsbaarheid in verschillende delen van de rivier? 11. Welke criteria zijn volgens u belangrijk bij het beoordelen van opties/maatregelen? 12. Hoe kun je ervoor zorgen dat verschillende stakeholders in het gebied uiteindelijk akkoord gaan met een bepaalde maatregel, draagkracht verkrijgen? Gebied zo inrichten zodat het aantrekkelijk is voor recreatie? -> denk aan klimaatbuffers van Gerard Litjens 13. Is het beter om te focussen op korte of lange termijn oplossingen? 14. Heeft u tips of data, relevante literatuur?

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b. Interview and trp Spui – Gerard Litjens - Stroming

Date: 23 April 2014 Place: Starting point Barendrecht Expertise: Director and co-founder Stroming By: Bas Schimmel, Femke Jansen, Corry Teichmann, Carlette Nieland

Summary ARK: climate buffer = recreational area + climate adaptation; more fresh water supply wanted (from Rhine). First this buffer should be realized (so be independent of the Old Meuse with respect to water supply + more dynamics should be created in the area). Only when this is realized, there’s more room for the Old Meuse to let it go more naturally: more tidal effects; this helps to not have the wash (golfslag) always at the same point on the banks (this is what Claus van den Brink from Arcadis also explained). The different viewpoints are shown in the map on page...... View point 1: Oude Maas - the only gateway to enter+ leave Nieuwe Waterweg + Spui = high volume of water, tidal dynamics and thus erosion - in past dykes were further away from river/ more space for rivers -> now dykes are closer to river due to landuse changed and agriculture+houses need the space (all 100 years new dykes are built); hence, little space for tidal energy; bricks and stones for needed for dyke stability = safety problem (to solve is goal of Deltaworks) where solution costs 0.5 bill. €. The biggest danger comes from the sea and you don’t solve this problem with creating more room for the river, but by creating strong dikes. To have strong dikes, you need strong forelands. And the erosion of these forelands that can be solved by creating more space for the river. - traditional method/ usually used as riverbank protection is concrete, stones and other hard protection measures where velocity is highest -> there are no other measures thought of because land is used/needed by farmers; but give river more land is also good solution. - around this area there are a lot of Natura 2000 areas! Hard protection measures are bad for ecology = other solutions needed-> e.g. restore original entrance Haringvliet - several landfills (polluted soil) close to Oude Maas and Spui bring problems to the system, unstable natural dynamics - difference of water level height in Oude Maas-> during droughts (low water level) = 50cm below sea level; +1m during flood - Oude Maas is the only river with tidal dynamics (some tidal dynamics are preferable, but not too much) - In past sedimentation in front of (old) dykes, which was sometimes taken away by high water levels (flooding) = nowadays these parts used as fertile soil for crops; sediments from Spui and Oude Maas go to riverbed of Biesbosch?! itself, in nature conservation part, harbor (20m³ dredging per year necessary) etc. - Two important points for this area = safety and ecology!!! View point 2: Oude Maas - About climate buffers = endure droughts by getting fresh water from Oude Maas and store it in green city areas - Priority is to have more space for tidal dynamics+ salinization; thus first restore fresh water dynamics; don’t open dykes - ARK: Ijsselmonde/climate buffers Rotterdam, polder ->there is opposition from people living there/farmers; is all done for environmental quality and recreation for Rotterdam people. Wim Dijkman is a representative for the agricultural community in the Deltaprogram. We can contact him. - Groynes 100 years old = no longer necessary and could be taken away; the wooden poles used to protect nature areas, to slow down velocity+ speed up sedimentation

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- Wanted: more variety in tidal levels/ bring tidal dynamics back, then we would have erosion everywhere! E.g. Haringvliet - Naturally 8m deep river; now 12m, but varying -> erosion deepens river bottom - At Spui it is like dike, riverbank, water; less navigation, everywhere agricultural use nearby - Best solution would be to move salinization upstream - 2 options to reduce Spui river velocity= 1) triple river width; 2) change Haringvliet management but: recreational areas are wanted, work together with local people!! - Spui shouldn’t get salty as fresh water from Spui needs to provide Rotterdam harbor with water - General hints for solution options: 1) not too small scale solutions; the size of the measures should fit the scale 2) combine functions in the solution, like with recreational stuff, fisheries, leisure… - E-mail Gerard again about getting contacts to farmers, essay/report , water directive framework - Gijs could give us an essay on what several nature conservation organizations work on at the moment - Companies involved: e.g. ARK, Staatsbosbeheer, Z-H landschap, WNF, Natuurmonumenten Viewpoint 3: Spui (with the fences we had to climb through) - Water to the Sea was made by ‘Stroming’ (a company Gerard was a co-founder of) and was payed for by the WWF. This water to the sea-strategy is in favor for the Harbour of Rotterdam, water supply for Groene Hart and a change of Haringvliet management is needed. But the waterboards and agrarians are hesitant because they don’t like it to be told what they should do differently. - Along the whole Spui there’s rock protection. There are 2 options to change it: o To reduce velocity (and thereby erosion), you need 3x as much room for the river. o Or you need to change the Haringvliet management. - When installing Water to the Sea program/strategy, the erosion in the Spui will be less. But it is hard to model actually the sediment transport. - Main solutions to combat the erosion in the Spui: o Climate buffer -> so what needs to be done in-dike -> fresh water strategy o Water to the Sea – strategy - Higher flow velocities than 40 years ago (when the storm surges and dams were installed). The bottom is almost 25 meters deep now in the Spui. The dikes are not strong enough! With storms you’ve higher velocities of course and thus more erosion. The water level difference is about 90 cm. This is strongly influenced by the sea. With storms it can be even 1 m more in difference. - Data about velocities we can probably get from Rijkswaterstaat (Pieter Biltman). Maps we can get from the Deltaprogram website or province of Zuid-Holland. - Along the Spui you find marine sediments. In the whole Delta most of the sediments are of marine origin, only 10% is from the river. Viewpoint 4 – Spui (near the ferry) Viewpoint 5 – Spui Viewpoint 6 – Spui (inlet to Bernisse) - Letting water in from Spui to provide fresh water to the harbor - That’s why the Spui should stay fresh!! - Spui is fresh because of closed Haringvliet. When the Haringvliet will be opened, the salty water can come until the line Middelharnis-Spui. - There is a need for sediments in Haringvliet (zandhonger) - Now the erosion is always at the same location of the banks (not a lot of tide) Viewpoint 7 – Spui monding (near Haringvliet) - A nature area is developed here very recently. - It’s a project where Natuurmonumenten is involved. - A channel on both sides of the Haringvliet is developed to supply fresh water to the achterland. This needs to be done before the Kier can be realized. - Piershil wants to have a connection again with the Spui (historical reasons) Answers to some questions

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- Maps: have a look at South Holland website and Deltaworks - Get flow velocity data from Rijkswaterstaat - Usually Spui differs in water level height around 190cm; for storm flooding even additional 100- 150cm with a strong westwind

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c. Interview Claus van den Brink – Arcadis

Date: 25 April 2014 Place: Arnhem Expertise: Consultant river engineer By: Femke Jansen and Larissa Gunst

Summary Now Claus van den Brink is involved in another project were the levees are overgrown by grass (Ijsselmeer area). There is not 1 solution for over the whole length in this specific project he’s talking about (Ijssel river), but he argues the Spui can/should be seen as a whole system. This is because everything in the Spui is now man-made. Morphology of the area is important, before one can decide which solution is best. Claus van den Brink thinks biocement is a very interesting solution, because it can be injected in the soil and this causes it to consolidate partly. Only a small part will erode. It should first be investigated with a pilot study, to find out the risks. However, it doesn’t work for fine sands. Rijkswaterstaat wants to do this, but it takes a very long time to get everyone involved. Other idea: put wood of willows on the floor, with in combination with biocement so it sticks together. We have to make use of the clay layers already present in the Spui area (not ‘normal dry clay’ from elsewhere) . This makes the consolidation process go faster. Willows project done in Kerkewaard. Margriet Schoor from Rijkswaterstaat is involved. Gerard Litjens mentioned that planting willows increases the roughness. Claus van den Brink thinks in the Spui it could actually be a solution because here the sea is dominant and not the river discharge. When the Haringvliet is opened the stones might be still needed, but because of all the feedback loops that might occur, it could be the case that the stones are not necessary anymore. Willows usually are a problem for shipping (no good vision in the corners), but since there is no ‘serious ‘ shipping in the Spui we can still use it. Claus van den Brink supports us to think out of the box (e.g. Daan Roosegaarde, TU Delft, who invented a clean-air building in Peking) He also thinks that short-term solutions are actually more sustainable. We should not aim for solutions of which we want it to have for 100 years. He also likes to think from the perspective of what people want or dream of to do in a specific area. From there you start developing technically the construction. Ownership of properties is very important for e.g. farmers. Criteria (he would probably use more than 5 criteria): - Safety/accuracy/reliability - How are you going to use the area  functionality - Construction and management  costs We have to come up with an idea that the inhabitants and thus probably also the Minister benefits from this solution (or at least not have to suffer). He really recommends to talk to Marja Hammilton (Arnhem) or Joyce Zuijdam of RWS. They know a lot about self-sustaining rivers. They like to think outside the box. Tel.: 026 3688911. A multifunctional dike is a very good solution he thinks. Example in Tiel. Regarding the vegetation: Claus van den Brink would not be reticent in using gene manipulation We should be aware that using non-native species could lead to overgrowing of other species. In addition, he recommended we should be aware of the temporal scale of the solutions we come up with. Using clay which is dredged from the rivers could actually be a very interesting solution to put on the riverbanks. When the clay dries it becomes very hard and strong, but he’s not sure about how long it takes to dry and if it is expensive or not.

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d. Interview Marjolein Meerburg & Hans de Bart – Waterschap Hollandse Delta (WSHD)

Date: 29 April 2014 Place: Ridderkerk Expertise: Hans de Bart: Technical managing project Zettingsvloeiing Spui Marjolein Meerburg: Stakeholder manager By: Larissa Gunst

Summary Most boats in the Spui are for recreation. Opening of the Haringvliet: salt water until Middelharnis/entryof Spui. The Spui is a tidal river (switch every 6 hours) with high velocities (5-7 km/hour) Problem by erosion: gullies are eroded. WSHD is filling the gullies again (Only the ones that threaten the stability of the dikes), for protection of the dikes. Protecting soil of the river for the dikes (to prevent ‘zettingsvloeiing’), not the banks. At the bottom of the river is mostly sand and clay and at the banks mostly blocks (breuksteen). Ecological top layer on top of blocks, but not used that much. Most important is the protection for the dikes. Blocks are expensive, so only used in e.g. tidal rivers, where only the dikes are not enough. On the floor of river there is mine stone (from Germany) and ‘breuksteen’ on top. One kind of species of crabs was not able to find their habitat on the riprap. Interests of RWS are dominantly for the depth of the river, and for WSHD the protection of the dikes. Bio-cement is not used by WSHD and Hans de Bart hasn’t heard of it before. Marjolein Meerburg stated ‘’we do not use cement for laying bricks’’(metselen). No other plans for decreasing of erosion are discussed. Erosion is a natural process and if we stabilize it on one place, another place may erode faster. Bricks will move due to high velocities of water.

Small opening of Haringvliet (kier) will not affect the erosion process and water speed as such, since large amounts trough the Spui are still needed for e.g. protection of fresh water close to the harbour. Recommended to have contact with Pieter Beeldman from RWS (Boompjes in Rotterdam) (kierbesluit) Farmers and inhabitants have their needs, and water in the ditches will be kept on the right level by WSHD. WSHD doesn’t take care for recreation, more important for the province of Zuid-Holland.

Regarding stakeholders, Rijkswaterstaat and municipalities are more involved (do have an influence on the plans). Other stakeholders, e.g. inhabitants, are informed about the plans but don’t have a direct decision. WSHD haven’t had any complains, only a few questions from the locals. The situation and location of the Spui is either quit simple and therefore big problems with inhabitants were not expected. Criteria for questions to stakeholders: - criteria depend on the owner of a certain part of the area (RSW, Waterschap, staatsbosbeheer, urban area etc.) - Natura 2000 and beleidslijn grote rivieren have to be checked first - for Waterschap: safety is the most important criteria - damage for people and nature - environmental quality (ugly/beautiful) - reachable: can I still get there? - makeable, not too much in a dream

Recommendation for contacting stakeholders: call them first to find the correct contact person and some farmers might have no internet. Contact Hengelsport Midden Nederland & Sportvisserij Zuid-West. Not a lot of farmers are in contact with the Spui. Municipality of Oud-Beijerland and their role  Directors, permitting authorities and owners Ask organizations close to the interests of the commissioners of this project as well as very different (not nature) companies.

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e. Interview Arie Broekhuizen & Ary van Spijk – Rijkswaterstaat Date: 30 April 2014 Place: Rotterdam Expertise: River adviser, water flow management, morphology, salt intrusion and river hydrology By: Bas Schimmel & Larissa Gunst

Meeting at Rijkswaterstaat West-Nederland Zuid; Arie Broekhuizen; river advisor, water flow management, morphology, salt intrusion and river hydrology and too less and too much water. Arie Broekhuizen, is going to replace Ary van Spijk in a few days. In the Spui area, only disposal rocks (stortstenen) are used to prevent erosion, no other measures because of the protection of the dike, that is the main issue in the area. The disposition is done by the water board Hollandse Delta. They are also concerned with dike protection in the area. The “wet” part of the river is managed by Rijkswaterstaat. This may cause problems because the dike ownership lies with the Water Board and the management of the river in total is done by RWS. This means that the erosion problems are detected by RWS, but must be taken care of by the water board. The Dutch central government pays for 50% of the dike maintenance the other 50% is paid by the water board (which is also subsidized by the central government; the majority of the costs is therefore for the central government).

There is dealt with the consequences of climate change through the national delta plan large rivers. Climate change and connected with that, safety, are considered until 2100. The erosion problem is observed and the delta program is notified; the latter looks into the problem at the moment and they keep track of it; if necessary, they are also the ones that intervene in the river. The expectation for the water in the Spui after the opening of the Haringvliet is that it is not going to change very much. Since the delta plan of 1953 the closure of the (with scheepssluizen) and Haringvliet (with Spuisluizen) was of importance for Rijkswaterstaat. The program is programmed on a way that the sluices open and close as a consequence of the inflow of water of the Rhine. This is measured at the border with Germany at Lobith. 1500 metric cube of water is sent to Rotterdam to keep the salt water as much as possible out of the estuary of the Hollandse IJsel river. Since the Haringvliet dam is in use, there is erosion in the Spui. There cannot be done much to prevent this erosion, possibly only to open the Haringvliet dam completely. In the old situation, the Spui was balanced, but currently not anymore because it is used for the water outflow of the Rhine in the direction of the Haringvliet. In addition, filling and loading the Haringvliet takes place in this tidal river.

If the area is restored naturally, the haringvliet needs to be able to manage 18000 cubic meters and stops erosion in Spui. The maximal capability of the current sluices in total is 6000 cubic meters. This is therefore not enough to supply enough water, the Haringvliet is then still being filled up from the Spui, causing erosion. If more water from the Haringvliet is permitted, it is always the question if this would promote sedimentation in the Spui. The build-up of the bottom of the Spui is sand, peat and clay deposits. Solutions for the problem are, next to the open haringvliet, to cover the entire area with asphalt, stones and bricks. This will keep the base of the river in its place for the coming decades. The development of biocement is not ready yet, there is research done but it is uncertain how it can prevent erosion. A mixture of clay and cement would be an option, but the durability of this solution is also not clear. There are uncertainties related to the streaming velocity of the water; does it still stay in place at high velocities (as in the Spui). If you are going to use this, you use the cement to fill out the holes in the base of the river; it is like putting a lid on a jar. Around the edges of the lid you will get more erosion, since this is now the weak- point. To be able to use this efficiently, you probably need to cover the entire river with cement. This would be a “draconic measure”. If the Haringvliet dam is removed, this would be the solution for preventing erosion and probably promoting sedimentation, however this causes problems for the safety standards of dikes, they must be replaced since these are then sea-dikes. Another problem is that there will be salt-water intrustion until the Biesbosch, which is used for the storing of fresh water now for e.g. drinking water.

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This fresh water is used for drinking water, industrial and agricultural use. Removing the Haringvliet will thus result in other problems. Financial aspects play a role here, because all measures necessary to work this out would be really expensive. The entire water scheme (waterhuishouding) of the Netherlands must be adapted if the Haringvliet dam will be removed; this is a major disadvantage. Regarding the stakeholders; Participation by the community is done through regional steering groups, river days and info evenings. The regional planning core decision is made later (Planologische kernbeslissing). The legal co-decision procedure can then be walked through after which the implementation is done. Professional and recreational navigation, agriculture are also stakeholders, however, the professional navigation is not that important in the area. Recreational navigation is important. The interests of the first group might therefore be less important in the Spui.

Criteria Cables and leidingen running beneath the river basin will be made vulnerable if the river erodes. Contradiction of nature versus agriculture Ecological quality: If you use disposal rocks or cement or the like for the entire river basin; is it still a living basin?

Alternatives Close of the Spui and redistribute the water over the entire area. There is more water pressure in the direction of the Hollandse Waterweg, which would also prevent salt intrusion from the Waterweg. The problem with this solution is that erosion will be even heavier in the Dordtsche Kil. The closure can be done in two ways, permanently (by damming) and not permanently for instance through the use of a “balg-stuw” in the river. The deposition of rocks is not that expensive. It will cost you the first time if the rocks are deposited but then the rocks stay there for about 50 years. Maintenance costs are not that large, however you must monitor the area and if necessary take precautions or other measures. If it is done well, this will not cost anything in the first couple of years. For instance, at the Dordrechtse Kil, the Water Board also deposited rocks last year; it is not necessary to look into that now anymore besides the regular monitoring. The regulation KWA++ says something about the system in which via Amsterdam Rijnkanaal water will be transported to the west of the country. This is part of the delta programme. The water boards adapt the gemalen and the like. At the eastern side of the Spui area, the water can be pulled into the area, this has a consequence that the Hollandse waterweg will have a larger risk of salt intrusion, which we must accept. (This is also done to make the country less vulnerable to changes in river flow through climate change). Short or long-term thinking? Deposition is cheaper if this is done with less rocks or sand, but this would be gone after a year in the Spui area. Solution is deposition with larger and heavier rocks, which would in the end be cheaper. Deposition of rocks would, in general, be for the long term, otherwise the national stress test of water defence systems would be failed (toets waterkering). If you would cover up the patches (like with biocement) this would probably be short-term because erosion then takes place around the patches. The preference would go to the cheapest solution for the coming 50 years. For the testing of dikes if they meet safety regulations long term solutions would be better. The testing is now done every six years, but in the future this would be every 12 years. If money would not play a role there are two options. The one is to completely protect the river with hard measures. The water board would prefer this for safety conditions, however this is ecologically not a very good option. The costs of the entire plan are not yet estimated. The other option is to get rid of the Haringvliet dam completely. All dikes must be heightened then, fresh water supply must be done at the Biesbosch where extra “spaarbekkens” must be created. The last option is blocking the Spui, although this would mean that the problem is not at the Spui area anymore but, as a consequence, the rivers in the area would have more erosion so the problem would be still in place. Quote: “We have to accept that we are done building in the Netherlands” Ask Pieter Beeldman for the Kier decision.

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f. Interview Jan Kruijshoop – Rijkswaterstaat

Date: 29 April 2014 Place: Wageningen Expertise: Senior Principal Advisor RWS By: Bas Schimmel and Carlette Nieland

Summary Are the current hard measures being monitored? Yes, we monitor the rivers and the effectiveness of the measures. We can ask Wilfred de Zeeuw for the costs of maintenance and management of the rivers. Langsdammen (such as those that are alongside the ) can prevent erosion because they will decrease the speed of the water. The regular dredging programmes in the Gelderse bocht have gained some ecological benefits, however there is much more space alongside the Waal then alongside the Spui River. At Low River tides the water quantity running through the Lek River is also much lower (300 cubic meters) than the quantities running through the Rhine (800 cubic meters). During periods with water shortages, currently the river water is diverted via the Waal to the western part of the Netherlands. Nature organisations relate out of the system approach (systeemredenatie), whilst Rijkswaterstaat tend to relate much more from out of the safety aspect. Currently, the task for the dike systems is now to repair spatial planning errors made in the past and preform large-scale maintenance. Shipping plays an important role. The stability of the shipping groove is important; this is why there are thresholds (drempels) in the rivers to promote this stability. This is done for instance by Driel (next to Arnhem), however this is problematic in the dry periods when the water table is low; ships can then not navigate through the rivers. Shipping is managed by the Beheersplan voor de Rijkswateren that is connected to the waterplan of the ministery. At the Spui, there is probably not enough room for eco-engineering. This is in contrast to rivers where the room for the river program runs. At the Spui, there are not many river forelands (uiterwaarden). Room for the river is not a profitable scheme; it is solely for the security of the delta and the sustainable use for fresh water (in first instance for shipping, then for industry and commercial use and household use), lastly the ecological aspect of fresh water is important. Het Spui is “een afvoergoot van jewelste”. - The Spui is a major discharge chute The ajar decision (kierbesluit) finally is made, while no one was expecting this. Sometimes change is possible, however this depends on many factors. At Rijkswaterstaat, there used to be a lot of ecologists (even more than at the department of agriculture), but the time was not there for a change. Now, the Haringvlietdam is partly opened because of a lost litigation; fish migration is the most important factor in this decision. For our research, it is important to include these kind of political societal history issues.

You can only consider plans for the delta as a whole. The region is large; small-scale plans might not be the most efficient. Sometimes, plans at a small spot are not preferable; it might be that at other places much more (ecological) profit can be made. At RWS there is a look from the technical side and the decision is ultimately a cost-benefit analysis. De minister in the end gets a table with +/-/--/0 so that she can make a decision for a region. This is usually done of cost-effectiveness and (mostly) not on the basis of ecological values. Costs are most important, especially in an economic low period. The costs for the replacement of the dams etc. (Natte kunstwerken) are estimated at 500 million for next year. The Maeslant barrier can be used for a while. The Harbour of Rotterdam is claiming more room from the sea. Maasvlakte 3 and 4 are already taken into consideration. With less water flows because of climate change, navigation must be handled by smaller ships. The ocean ships, on the other side, become increasingly larger, which also makes the expansion of the harbour necessary. Jan asks why the harbour would need fresh water, ships can still float on salt water. (Note by author, this is probably for businesses and industries that the harbour wants to improve diversification)

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Greenhouses have a more independent water supply because they don’t want to be dependent on fresh water from unreliable sources (as could be the case in summer for instance). Stability of peat dikes can also be done with salt water, so if the companies do not rely on fresh water, it might not be so disastrous if there is salt water in that area. A sustainable inlet can also be made via the IJsselmeer. Transport of fresh water can then be done by transferring it from the IJsselmeer to Amsterdam. The IJsselmeer then acts as a buffer for dryer periods. It is unknown how the development of land use that dependent of fresh water is. A problem with the fresh water in the region is that much of it flushes into the sea via the nieuwe waterweg en adam/rijnkanaal, noordzeekanaal. Other places then the Spui might be a better place for nature conservation projects, such as which can be added to the estuary dynamics this is comparible with the Volkerakmeer. This seems more profitable than that small piece of Spui that can be changed. Rijkswaterstaat looks at tourists in Zeeland; blue-green alga might there be a larger problem than the fresh/salt water problems.

Participation by the community is done through regional steering groups, river days and info evenings. The regional planning core decision is made later (Planologische kernbeslissing) The legal co-decision procedure can then be walked through after which the implementation is done. Comparable situations: Integral management plan Eems-Dollard. The area is comparable, small river with Natura2000 around it. Recommendation to look at the large scale for solutions of the problems. The scale of the Spui is probably too small. Look also at navigation, independence of fresh water by the companies that now depend on it. A long term vision is necessary because the dams, barrages and weirs need to be replaced in the coming 20 to 30 years. Recommended contacts for further talks: Toine Smits; instuut duurzaam rivierbeheer -> Ander systeem van vervoer; “River snake” kleinere boten? Bas Jonkman van de TU Delft (dat is de opvolger van Han bij de regionale stuurgroep) Bas van Hurk: 26 mei KNMI; nieuwe klimaatprojecties. Contact with Toine Smits. Useful references: http://www.rijkswaterstaat.nl/water/plannen_en_projecten/bprw/ https://deltaprogramma.pleio.nl/groups/profile/69902/staf-deltacommissaris

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g. Interview Tom Wilms and Steven te Slaa – Witteveen + Bos

Date: 01 May 2014 Place: Deventer Expertise: Both studied civil engineering at TU Delft. Tom did an internship at Staatsbosbeheer to get more involved with ecology. Steven specialized in coastal engineering, morphology, coastal erosion and focused during promotion on fine sediment dynamics, soft engineering and mangrove processes. By: Femke Jansen and Carlette Nieland

Summary They often at Witteveen+Bos use the same technique with solutions which are evaluated using Multi- Criteria Analysis. Sometimes a small adjustment of the solutions will help to get the approval of stakeholders. So it’s important to understand the whole system with stakeholders, legislation and environment and their coupling.

1. How useful is it to solve the erosion of the riverbed. You can change the flow area and put gravel or concrete blocks there, but this can lead to erosion problems elsewhere. Other softer options could be: - Putting tree trunks along the river on the banks to prevent erosion. It will create more backwater (how much?) but it creates on the other hand more space for habitats. - Pieces/mattresses of sink with willow sprouts; self-sustaining banks - Mattresses of willows - Use the wastes of other nature areas to put on the banks as a protection measure, e.g. willows removed in one place, put on the banks of the Spui. Benefit for several parties. Question remains how much maintenance it will take and to what extent you translocate the problem. The solution also depends on what ecosystem you want to have in the end, should it be dynamic; then a clear dynamic in water level is needed. Note that erosion still can take place above the (stone) protection when the water levels are high.

We should include the base case (situation as it is now) in our research/evaluation. 2. When you buy a large area around the river there’s more opportunity to create recreational areas which might give more support from society. 3. Donau river also needed to be broadened. They used bumps/barriers. Often in the end hard measures are taken because companies know they work. So more pilot studies are needed on soft measures to provide prove for the functioning of these soft measures. Now you never know for sure if it’s going to work, this gives rise to conservative/hard options instead because they already proved to be efficient. Hard measure options: Use of gravel to protect the riverbed Use of stones but with more texture and gaps which could be beneficial for flora and fauna to develop 4. B-shore approach could be a good opportunity to try in the Spui; a lot of potential. They’ve done already quite a few experiments with this concept (5/6 years). They’ve proved to dampen the waves and to retain sediments. It forms a very stable layer on top of the riverbank/bed. You could use it in combination with geo-textile beneath it. 5. Make use of stones with structure and with gaps in between them to provide sheltered areas. You could chose to make some parts along the river more dynamical to get more diverse nature development. 8. It is important to involve farmers. They can be benevolent if they are allowed to speak up and if they are getting involved in the process of posing possible solutions. It’s best to do this in the early stage of the process. Often they can provide good ideas/solutions, even better than you would have thought of yourself.

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11. We should think of large scale solutions. Changing the hydraulic boundary conditions (lower the discharge)? Improve sediment balance (redistribute sediments)? Tackle the banks along the whole river? 12. Criteria (about 5 – 7) - Cost/benefit - Safety - Experience (beleving) - Coupling of multiple projects; sediment balance (use the waste of 1 project for the other project) - Maintenance/construction/lifespan/costs (don’t necessarily have to include costs in this) - Scenery/landscape/esthetics

Involve stakeholders in choosing the criteria and the order and valuing/weighing of the criteria. Sensitivity analysis would be good to perform. From higher up decisions have been made that the ecology is an aspect that should be improved along rivers. So it’s better to already start incorporating that in the solutions that are created now, otherwise later on improvements on these solutions should be realized and this will cost extra money. There might even be extra financial support systems when working on increasing the ecological values. No regret measures – everything you execute should not have a negative impact later on. So you need to keep up with the developments that will occur later. This means that you can come up with a solution for the next 10 years, and after that you come back, evaluate it and take new measures. Disadvantage of this is that you’ll need money again after those 10 years. Or you simply do nothing. This is also a solution. You buy out farmers and let the nature develop. But what will this ‘doing nothing’ cost? Recommended literature: - Keurhandleiding natuurvriendelijke oevers – 5 boekjes - STOWA – handreiking natuurvriendelijke oevers; een standplaatsbenadering - Wiki pagina van Building with nature – EcoShape – Erik van Slobbe (case manager) - Grondbank.nl or grondbalans.nl - Ruimte voor de rivier - Deventer.

Recommended contact and reference: Maarten van der Wal – Deltares (Markermeer) NMIJ – project Markermeer + Ijsselmeer Further remarks Send the report (already draft version to check if we used their information correctly) Inform about possible discussion meeting stakeholders + presentation at the end

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C. MCA

I. Scoring In the following tables we give the reasons for certain scores of erosion protection measures in the MCA. To shorten it, abbreviations are used for the single measures: GC = Ground Consolidators, WM = Willow mattresses, WR = Widening the river, BR = Blocking the river, OH = Open Haringvliet and KCS = Keep current situation.

Table 1: Reasons for scoring sub criteria in the group of “Diverse land use”-criteria in the MCA

Measur Recreation Navigation Agriculture Housing Nature Drinking water supply GC -; No secondary 0; No changes -; Less space 0; No +; Stones 0; No functions possible, for grazing changes removed, changes not allowed to vegetation gets enter the banks in chance to comparison to develop (little situation now bit more space than before) WM -; No secondary 0; No changes -; Less space 0; No +; Stones 0; No functions possible for grazing changes removed, changes not allowed to vegetation gets enter the banks in chance to comparison to develop (little situation now. bit more space than before) WR ++; More space for +; More space for --; Takes land --; People ++; More space --; No fresh recreation and ships to pass from have to for nature on water basin recreational each other agriculture move the banks of the anymore, shipping, lower river, more which is velocity of the space for the currently water river there BR +; Lower velocity -; Using river as 0; No change, 0; No ++; More space -; Pipeline improves quality connection to maybe water changes for nature at for fresh for swimming; other rivers is quality places where water Because dikes are not possible decline but dikes are necessary not necessary we anymore for assumption is removed create space for ships to have still other functions; But fresh water recreational supply shipping will be limited OH +; Opening the 0; No change, -; Efficiency in -; People +; More -; Chance Haringvliet is good small decline of agriculture have to opportunity for that salt for fish migration tidal dynamics might be move but nature, but water from sea, fishing which might be lowered due can be depends on land intrudes in possibilities positive but since to salt water relocated use planning of the area. therefore increase. river is deep, not to the area climate dikes Climate dikes too important later maybe mainly used for agriculture. KCS 0; No change 0; No change 0; No change 0; No 0; No change 0; No change changes

Table 2: Reasons for scoring sub criteria in the group of “Socio-economic criteria” in the MCA

Measure Esthetics Local support Landscape value Economic value GC 0; Not sure how it will +; Not much change for 0; No large change 0; No change look like in the end people (positive); but new technique = people might be sceptical WM +; Change from bricks ++; Not much change, +; Instead of stones, 0; No change 94

to willows more bank stabilization there are trees and experience in the solution. WR ++; Natural look of the --; Much space lost for ++; Much more space -; Agriculture declines, river, beautiful nature and other for a natural system and but recreation improves landscape landuse nature BR ++; More space for 0; Equal negative and +; Using river as -; No through shipping nature without dikes, positive arguments connection to other possible, natural looking rivers is not possible landscape anymore; Less natural dynamics OH +; Climate dikes are -–; Too many +; Climate dikes are -; People have to move, more beautiful uncertainties & people more appealing than lost revenues from (depending on land use have to move dikes present now agriculture due to plan) climate dikes KCS 0; No change ++; No changes for 0; No change 0; No change people necessary, no higher costs involved

Table 3: Reasons for scoring sub criteria in the group of “Ecological value”-criteria in the MCA

Measure Increase Stop salt Restore tidal Prevent erosion, promote biodiversity intrusion dynamics sedimentation GC +; no stones 0; No change 0; No change +; Roots in the soil that prevent anymore & more erosion, reduces wave strength chances for plants to and hence promote establish sedimentation; But vegetation takes a bit longer to establish WM +; see GC 0; No change 0; No change ++; See GC, but it starts working already after a few days WR ++; Lower slopes 0; No change +; Wider river ++; Less erosion&more possible; more assuming same sedimentation as water velocity gradual transition volume of water, so decreases (same amount of water less dynamics but larger river) BR ++; See WR ++; No possible for –-; No tidal dynamics ++; No constantvelocity of the the salt to enter in water, so less erosion&more Spui sedimentation OH ++; More fish and 0; No change ++; Yes tidal dynamics +; Decreasing erosion because of thus more birds & tidal dynamics, less water volume nice climate dikes passing, Uncertainties on sedimentation KCS 0; No change 0; No change 0; No change 0; No change

Table 4: Reasons for scoring sub criteria in the group of “Costs”-criteria in the MCA

Measure Installation costs Longevity Maintenance costs GC ++; Small scale, relatively cheap +; Vegetation takes over after a ++; No maintenance costs while WM ++; see GC +; Vegetation takes over +; Pruning needed, but still relatively low in costs WR --; Land-owning problems, land ++; Long time frame +; Might be necessary to dredge; needs to be bought dikes need maintenance BR 0; Not very cheap but also not ++; Long time frame +; Only maintenance costs for very costly dams OH - -; very expensive ++; Long time frame 0; measures in the river such as blocks still needed, however this is less because velocity of water is decreased KCS ++; nothing needs to be installed, --; Dikes and bed need to be put -; maintaining stone riprap is 0.5 thus “cheap” in stones billion € per year in Spui area

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Table 5: Reasons for scoring the criterion “Safety” in the MCA

Measure Safety GC 0; No change WM 0, No change WR ++; Much safer due to less velocity BR ++; Much safer due to no velocity OH ++; Much safer due to less velocity KCS 0, No change

II. Ranking

Figure 1: Result of Multi-Criteria Analysis: Stacked histograms showing ranking of erosion protection measures and individual contribution of group criteria

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D. Open answers questionnaire (Dutch)

Hoe bent u betrokken bij de mogelijke (her)inrichting van het gebied rondom het Spui?

Als bestuurslid waterschap, als voormalig recreant (zeiler), als bioloog/ecoloog Nee Niet Als lid van het algemeen bestuur van het waterschap en als inwoner van de . Reeds rond 1985 herinrichtingsplan Spuigorzen ingediend bij prov. Zuid-Holland i.v.m. prijsvraag. Daarna overleg en veldwerk met RWS en later Deltanatuur. Nu ook via project dijkversterking Spui-oost. Voorts door het bepleiten van spoedig estuarien herstel van Haringvliet, dus ook Spui o.a. in kader Deltabeslissingen ZWDelta. Geen directe betrokkenheid; meer in zijn algemeenheid als natuurbeheerder. Omwonende Via het afstudeerproject van Tom Smits (TUD Civiele Techniek) over de oorzaken van de erosie in het Spui en andere delen van het Noordelijk Deltabekken en wat daaraan te doen is. Nog niet Als bestuurder waterschap Ik ben niet betrokken - Werkzaam bij Rijkswaterstaat, - Dorp stichting "Swaneblake" Piershil. belangenbehartiger voor natuur en landschap en (mede)initiatiefnemer Als recreant Ik heb een paar jaar geleden meegedacht over de mogelijke inrichtingen van Spui-West. Focus: waterveiligheid, recreatie en natuur. Het was een eco-dynamische ontwerpen studie. Ik heb er een groot deel van mijn leven in de buurt gewoond. Het Spui heeft een sterke samenhang met het Haringvliet. Voor dit gehele gebied is het belangrijk dat de getijdenwerking toeneemt en de geleidelijke overgang tussen zoet en zout wordt hersteld. In tegenstelling tot bijvoorbeeld de Grevelingen, valt er veel te winnen. In de regio is een erorme oppervlakte aan getijdengebied verdwenen, niet alleen langs het Haringvliet, maar ook langs bijv. de Oude Maas. Hoog tijd voor getijdenherstel! Ik ben betrokken bij studies naar de mogelijkheden voor het verder openen van het Haringvliet om daar de estuariene dynamiek te vergroten. Bewoner via discussies over combinatie van veiligheid en natuurlijkheid in het gebied via zetel in VV waterschap Als bewoner en natuurrecreant van de Delta. En soms als ecoloog bij het opstellen van toekomstvisies. ecologisch betrokken als vogelonderzoeker voor rijkswaterstaat niet direct, indirect via natuur- en landschapsvereniging Ideevorming en NGO dit natuur wil herstellen. Watersch hollandse delta Vanuit lokale politiek mbt tot veiligheid, recreatie en economie niet Bewoner van de Hoeksche Waard en dagelijks langs de rivieren actief. Via stichting Swaneblake, welke een medisch georienteerd'recreatieplan heeft ontwikkeld met de gemeente' , gesitueerd aan het Spui. ik ben lid van het Algemeen bestuur Waterschap Hollandse Delta

Over welke effecten van klimaatverandering heeft u gehoord?

Stijgende zeespiegel, dalend land en daardoor het verder indringen van zoutwater en getij dan wij nu gewend zijn. Hogere zeespiegel, wat leidt tot grotere zoutindringing via de Nieuwe Waterweg. Lagere rivierafvoeren, met name in de nazomer, waardoor er onvoldoende zoet water beschikbaar is om zout water tegen te houden Anders stormklimaat waardoor met name in het winterseizeon er grotere hoeveelheden zoutwater in het Haringvliet terecht komen, die dan lastig terug kunnen stromen naar zee. De rivieren krijgen steeds meer te maken met extremen, qua droogte maar zeker ook qua piekafvoeren. Dat betekent dat er tussen dijken die absoluut veilig moeten zijn, er echt meer ruimte moet komen voor het water ('Ruimte voor de rivier'). Het alleen maar uit zijn op het zo snel mogelijk afvoeren van water, leidt tot steeds gevaarlijkere situaties. Meer waterafvoer vanaf de bovenlopen. afvoer van regenwater na hevige buien. afname van biodiversiteit door opwarming en verdwijnen van biotopen. Verhoogde bovenwaterafvoer. Extremen in het weer zullen vaker voorkomen waardoor cumulatie van van deze extremiteiten tot gevaarlijke situaties kunnen . hogere en frequentere hoge waterstanden Versnelde relatieve zeespiegelstijging Hogere gemiddelde temperatuur. Extremere weersomstandigheden . Zeespiegel stijging. Zout indringing. opwarming van de aarde waardoor klimmatgrenzen naar het noordern verschuiven. verhoogde zeespiegel, vertraagde afvoer rivierwater, 97

verslibbing m.n. Haringvliet Water-afvoer gerelateerd Verzilting/verdroging extremer weer zeespiegelstijging Afhankelijk van daadwerkelijke zeespiegelrijzing op Noordoostelijke Atlantische Oceaan zal verzilting steeds moeilijker tegen te gaan zijn. Het zoetwaterbeleid zal steeds duurder en krampachtiger (moeten) worden. De temperatuurstijging zal steeds meer zuidelijker (mariene) soorten hier een kans geven. Wellicht komen er dynamischer neerslagperioden aan, naast langduriger droogteperioden. Zeespiegelstijging. Verzilting door verdere indringing van zout in het rivierengebied, dit kan leiden tot watergebrek voor landbouw en drinkwatervoorziening en een verandering in het type soorten dat kan voorkomen in dit gebied. Regelmatige(re) en langere periodes van droogte kunnen leiden tot watergebrek voor landbouw en drinkwater voorziening; droge periodes kunnen ook zorgen voor te weinig doorspoeling van Het Spui waardoor de waterkwaliteit kan verslechteren. Aan de andere kant kunnen langere warme periodes van hogere temperaturen de verdamping verhogen waardoor de waterbehoefte van landbouw sterk toeneemt. Door meer en vaker voorkomende stormen kunnen de rivieren belast woorden met hoge afvoeren, waarbij het water sneller kan stromen, waardoor de kans op erosie groter is. Dit kan effect hebben op de veiligheid. verzilting, hogere pieken in spui - (achterwaartse) verzilting en zoet- water inlaat Bernisse, - hoeveelheid (beschikbaarheid en teveel) water, - water kwaliteit (eutrofiëring, zuurstofloosheid, botulisme, blauwalg) - teruglopen van biodiversiteit, - erosieproblemen met daarbij de stabiliteit van dijken. Ik vind de effecten van klimaatverandering niet relevant voor het herstel van getij in het Haringvliet-Spui. Meer aanvoer water heeft impact op erosie Ben zeer sceptisch over klimaatverandering door menselijke invloed; is in mijn visie ordinaire bangmakerij c.q. volksverlakkerij zeespiegelstijging, meer neerslag hogere zeespiegel-> meer zoutindringing zelfs leidend tot achterwaartse verzilting lagere én hogere afvoeren van de rivieren zeespiegelstijging, hogere gemiddelde temperatuur, meer extremen in klimaat (piekafvoeren rivieren, droogte, hevige regenval), verandering plant- en diersoorten, Grotere perioden van regen en droogte De regen periode"s zullen korter maar heftiger zijn en de drogere periode's zulle ook heftiger zijn .

Wat zijn naar uw mening de belangrijkste gevolgen van de erosieproblemen?

Instabiele dijken, problemen met andere infrastruktuur en invloed op buitendijkse natuurwaarden erosie door verhoogde stroomsnelheid, in gevaar komen van dijken, verdwijnen van (ecologisch belangrijke) grondeninstabiliteit in het watersysteem, veiligheidsproblemen (onderloopheid, dijkval), kunstwerken die erosie moeten tegengaan die afbreuk doen aan natuurlijke processen, het zelf-ordenend vermogen, de biodiversiteit en de belevingswaarde. Mogelijk instorten van de rivierwater kerende dijken. Toenemende stroomsnelheden, weinig gradiënten, weinig natuurlijke oevers in het Spui aantasting van de oevers. Op het land bij droogte verdwijnen van de vruchtbare toplaag. (Zoals in USA) Stroming en inrichting van de oevers. Bijna kanaalvorming en geen uiterwaarden die als waterbuffer kunnen fungeren Het ontstaan van gevaarlijke situaties en het verdwijnen van buitendijks gebied. Destabilisatie van dijken met als gevolg een verminderde veiligheid in een gebied; verdwijnen van zandplaten waarop unieke soortgemeenschappen leven; grotere onderhoudsactiviteiten leiden tot hogere kosten voor rivieronderhoud (enerzijds dreggen, anderzijds verstevigen van dijken); zandverdeling in de rivier kan leiden tot problemen voor scheepvaart omdat andere delen van de rivier kunnen verondiepen. eb- en vloedbeweging, stroomsnelheid Het wegvallen van de natuurlijke dynamiek in de rivier, het vastleggen van de rivier dmv dijken en strekdammen en de scheepvaart. Afkalving van de oevers en sedimentatie op plaatsen waar de stroom minder is. ondermijning dijken risico op dijkval extra kosten om de dijken veilig te houden Haringvlietdam leidt tot erosie en zettingsvloeiing bij dijken Gesloten Haringvliet. - stabiliteit van dijken, - vrij komen van kabels en leidingen. De belangrijkste gevolgen van de erosieproblemen zijn: 1) de bewustwording dat de huidige rivierwaterverdeling in de toekomst niet houdbaar is. 2) de bewustwording dat we beter nu al kunnen omschakelen naar een andere zoetwateraanvoer dan de huidige, d.w.z. dat het zoete water uit het oosten gehaald moet worden en niet vlakbij de monding. 3) de bewustwording dat de huidige Deltawerken hun eindpunt genaderd zijn. De problemen die met de huidige Deltawerken en hun

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beheer samenhangen worden steeds nijpender. 4) teveel water door een smalle riviertak persen ondergraaft de oeverlanden en dijken die daar op staan. De oevers worden steeds steiler en de geul dieper door erosie. Uiteindelijk zullen delen van de oever (inclusief dijken) in de geul schuiven en past de riviertak op natuurlijke wijze zijn breedte aan op het debiet dat er doorheen stroomt. Uiteraard zal de mens proberen dat niet te laten gebeuren, maar dat is vechten tegen de natuur en dus altijd duur. Enorme kosten: In het Spui worden nu al maanden massale hoeveelheden steen gestort. Verlies aan natuurlijk habitat: kleibodems veranderen in steenbodems. Natuurlijke opbouw en afbraak kan niet meer plaats vinden (gevolg van te sterke stroming en afsluiting Haringvliet) Verstoring natuurlijke soortensamenstelling wegens de onnatuurlijk sterke stroom. Het Spui is een kanaal geworden (vergelijk o.a. de Seine, waar hetzelfde gebeurde!). Afkalving en verarming van de bodem Door erosie is de lastig erodeerbare dunnere bovenlaag doorgesleten en zijn nu finnere zanden in de ondergrond aan de beurt. Dit leidt in korte tijd tot diepe erosiekuilen. Dit kan uiteindelijk leiden tot het wegzakken van oevers en het instabiel worden van waterkeringen. Ook neemt op langere termijn de influx van water naar het Haringvliet toe, waardoor er ook makkelijker zout water in het Spui door kan dringen.Bodemerosie kan niet alleen leiden tot oevererosie en instabiliteit van de waterkeringen langs het Spui, maar ook tot het aansnijden van fijne lagen in de ondergrond die versneld eroderen en de problemen verergeren. Bovendien kan verruiming van het Spui door de erosie leiden tot het aantrekken van nog meer water, nog meer erosie, etc. (positieve terugkoppeling). Opmerking: Jullie analyse van de oorzaak klopt niet helemaal. Het gaat niet alleen om de omleiding van de rivierafvoer naar het noorden door de afsluiting van het Haringvliet, maar ook en vooral om de toegenomen getijstroming doordat het Haringvlietbekken nu elk getij gevuld en geledigd wordt via het Spui, de Noord, en andere noord-zuid verbindingen tussen Nieuwe Waterweg en / Haringvliet. De getijamplitude daar is weliswaar gering, maar de hoeveelheid water die ermee gemoeid is groot genoeg om in die noord-zuid verbindingen stroomsnelheden teweeg te brengen die erosie veroorzaken. In technische termen: het getijprisma in deze geulen neemt toe en de dwarsdoorsnede past zich daaraan aan.instorting van de Dijken Wegspoelen fundament van de dijken.

Oplossingen

Verbreden van de Rivier: Wat vindt u van deze oplossing?

Je moet dan echt de stroomvoerende breedte vergroten, niet (zoals onderstaan plaatje suggereert) alleen het hoogwaterbed. Als je de stroomvoerende breedte vergroot pas je de doorsnede aan aan de toegenomen afvoeren (rivier en getij!) en werk je met het getij mee. Mogelijk nadeel is dat het zoutbezwaar vanuit de Nieuwe Waterweg toeneemt, wat van belang is voor de zoetwaterinname vanuit het Spui (o.a. voor het Westland). In hoeverre dit het geval is zou met een model moeten worden berekend. Overigens is dat zoutbezwaar ook een majeur punt van aandacht bij het Kierbesluit voor de Haringvlietsluizen. Opmerking: Dit Kierbesluit herstelt in beperkte mate het getij in het Haringvlietbekken en reduceert daarmee het Spui-probleem. De Studie van Tom Smits laat echter zien dat dit niet voldoende is en dat maatregelen in het Spui nodig blijven. Nadeel: relatief hogere kosten en grotere complexiteit Voordeel: waarschijnlijk landschappelijk een aantrekkelijker oplossing nieuwe gronden moeten gebruikt worden om natuur in te richten, recreatie mag pas op de tweede plaats komen, en extensief zijn. De nadelen wegen niet op tegen de voordelen, de gevolgen van de erosie repareren is de goedkoopste oplossing en kost ook geen grond. Bespreekbaar maar niet zeker of het haalbaar is over de gehele lengte. Zal onderdeel kunnen zijn van pakket van maatregelen Klinkt logisch. Je moet er wat voor opofferen en het gaat veel geld kosten (wat er amper is). voordeel: 1)minder erosie en daardoor uiteindelijk minder kosten. 2) meer natuur lijkt me beter dan meer recreatie, dat is er al voldoende in de Hoeksche Waard. Nadeel: 1) hoge kosten voor de onteigening van de landbouwgronden. 2) ik verwacht veel weerstand vanuit de bevolking. Dit zou een goede stap zijn. Voordelen: lagere stroomsnelheden, natuurlijke overgangen land > water. Nadeel; vraag veel (kostbare) ruimte Een redelijk goede en effectieve oplossing. Punt is alleen dat vanwege 99

de grote hoeveelheid water die bewust wordt afgevoerd een groot gebied nodig is om het probleem geheel op te lossen. Ik ben geen deskundige, maar als je het bekken groter maakt, neemt dan de 'druk' van het zoete water niet zo veel af een eventuele zouttong verder binnen komt? Voordelen voor natuur en recreatie. Nadelen, grotere kans op eutrofiëring, zuurstofloosheid, botulisme en blauwalg. Niet afsluitbaar bij zout indring en moeilijk om een verontreiniging op te ruimen. Acceptabel dat er ruimte word gemaakt voor een betere waterafvoer. Ik juich ruimte voor de rivier toe, mits deze dan ook zo ingericht en beheerd wordt dat natuurlijke processen hun vrije loop kunnen gaan. Daarbij zou het graven van nevengeulen in Korendijkse en Benninger Slikken misschien een meerwaarde vormen. Beide gebieden kunnen wel een natte impuls gebruiken. ivm de kosten niet realiseerbaar Nadeel: dat er landbouwgrond verloren gaat. Voordeel: ik denk wel een van de effectiefste oplossingen Dit lijkt mij onwaarschijnlijk. De erosie wordt namelijk veroorzaakt door de trekkracht van het Haringvliet. Het haringvliet trekt aan het water dat via de Nieuwe Waterweg bij vloed binnendring; omdat het een open verbinding is en er een peilverschil is tussen het vloedpeil op zee en in het Haringvliet. Als je het Spui verbreed, zal alleen maar meer water naar het Haringvliet stromen. Ik verwacht dat de erosie dan eerder toe- dan af zal nemen. Totdat de rivier zo breed is, dat het gehele getijvolume van het Haringvliet er (makkelijk) doorheen kan. is veel te zwaar aangezet Kost heel veel grond, veel mensen verhuizen en emotie. Mooie kans voor grootschalige natuurontwikkeling. Alleen maar nadelen. Landbouwgrond zal in de toekomst steeds belangrijker worden Een uitstekende oplossing, die meer ruimte voor natuur en recreatie zal bieden en uiteindelijk ook minder onderhoud kost. Voordeel is, of kan zijn, dat er meer intergetijdengebied aan de oevers van het Spui komt. Dit is erg gunstig voor de nu zeldzame zoetwatergetijdennatuur in de regio. Nadeel is wel dat er dan meer getijdenwater naar het Haringvliet stroomt en er dus wat minder getij voor de Biesbosch overblijft. Dat zal op andere plekken tot een vermindering van op oppervlakte zoetwatergetijdennatuur (kunnen) leiden. Met een goede aanpak is de winst echter (veel) groter dan het verlies. Een ogenschijnlijk nadeel is de maatschappelijke weerstand en het kostenaspect. Het kostenaspect kan geneutraliseerd worden door te wijzen op: tijdelijke werkgelegenheid ruimte geven aan verdienfuncties (jachthaven bijv.), maar denk ook aan plan Swaneblake bij Piershil minder onderhoudskosten in toekomst (kost gaat voor de baat) De maatschappelijke weerstand (de Ramp 1953, verlies kostbare landbouwgrond e.d.) kan vertrekpunt zijn voor het openen van de ogen dat het op termijn niet anders kan. Dit laatste moet intensief en gelikt gebeuren. Voordeel is natuurlijk dat natuurherstel kan plaats vinden, er komt ruimte voor (versneld) estuarien herstel. Eventueel eerst gedempt getij (= 80 cm getijslag Middelharnis, 100 cm Biesbosch). Voorts kan er in combinatie met Haringvlietmaatregelen meer water opgevangen worden. Natuur: positief Waterveiligheid: positief Recreatie: onbekend Kosten: waarsch hoog Mooie landschappelijke oplossing. Nadeel worden de kosten. kost veel ruimte Voordeel is dat er ruimte ontstaat voor een meer natuurlijk systeem met mogelijkheden voor recreatie en andere economische ontwikkelingen . Het bespaart op onderhoud van de waterkering. Er is nu al voor tientallen miljoenen aan stenen in het water gestort. Nadeel is een groot ruimte beslag en het probleem in de Dordtse Kil blijft bestaan. Goede oplossing omdat deze gebaseerd is op de oorspronkelijke ruimte die de rivier nodig heeft en kansen biedt voor natuur en landschap. Vraag is in hoeverre hier draagvlak voor is bij agrariërs omdat zij grond in moeten leveren. Ben voorstander. We moeten de rivieren hun oorspronkelijke functie weer laten vervullen. Rivieren zorgen voor transport naar zee. Wij hebben ze ondieper en smaller gemaakt. Goed principe, a la ruimte voor de rivier plan dat zeker mogelijkheden in zich heeft Goede oplossing, mits goed uitgevoerd in samenspraak met omringende stakeholders en met inachtneming van grotere ruimtelijke schaal Voordelen: meer ruimte voor natuurontwikkeling op floodplains, meer mogelijkheden voor de natuurlijke dynamiek van regelmatig overstromen van floodplains, meer mogelijkheden voor recreatie en een verhoogde veiligheid (grotere afvoer), door multifunctionaliteit een kosten-efficiente oplossing nadelen: bevolking verliest woongebied, werkgebied, landbouwgebied en zal moeten worden gecompenseerd of iig uitgebreid worden meegenomen in het besluitvormingsproces. Dit is duur en kost tijd. Door het vergroten van de afvoer, kunnen upstream ook veranderingen komen in de waterdynamiek; dit kan effect hebben op waterbeschikbaarheid voor landbouw e.d.Voordeel: Er ontstaat meer ruimte voor waterberging. Ook ontstaat er meer ruimte voor ecogische processen en meer natuurlijk systeem. Nadeel: Het kan leiden tot pijnlijke situaties voor boeren en anderen, die plaats zullen moeten maken. Hierbij is goede

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voorlichting én goede compensatie voor wie moet verkassen, essentieel. In de Biesbosch kan het ook, dus waarom hier niet?! voordeel: minder erosie, afvoer rivierwater gewaarborgd verlies land

Open Haringvliet: Wat vindt u van deze oplossing?

Voordeel: geen grote investeringen, landbouwgrond kan blijven. Ook het herstellen van getijden vind ik een voordeel Groot voordeel is dat natuurlijke dynamiek herstelt wordt. Voordeel: Eindelijk herstel van getijdendynamiek en de zoet-zoutgradiënt! Dit is ook in lijn met internationale afspraken. Het wordt hoog tijd, dat we die nakomen. Nadeel: het leidt tot forse ingrepen verder landinwaarts. Hebben we inzicht in de effecten hiervan? Dit zal het probleem in het Spui denk ik niet volledig verhelpen. Goede aanvullende maatregel voor oevers en natuur (trekvis). Niet realistisch omdat het Spui zeer waarschijnlijk te vaak te zout (brak) zal zijn. Het Spui voedt de Bernisse, die het Brielse Meer op peil houdt. Het Brielse Meer is DE zoetwatervoorziening voor Voorne Putten, de industrie in het gehele havengebied, en het Westland. Een alternatieve zoetwateraanvoer is heel erg kostbaar. Evides Waterbedrijf heeft een noodinlaatpunt aan de Oude Maas/Spui, dat bij het openzetten van de Haringvliet niet meer als zodanig kan functioneren, waardoor de drinkwatervoorziening van een groot deel van Rotterdam en omstreken niet leveringszeker meer is. De Haringvlietsluizen gaan toch op een kier. Onderzocht moet worden of dit voldoende is. Zo ja dan is dat een bijkomstig voordeel. Zo nee lijkt mij deze oplossing niet toepasbaar. Een uitstekende oplossing. De voordelen zijn zo groot dat ze de nadelen ruim compenseren. Daar pleit ik al jaren voor, ook in Deltabeslissingenverband. Vooral boeren zetten nu hakken in het zand en krijgen er nog steun voor. Voor de afsluiting was het geregeld mogelijk in het zuiden van de Hoeksche Waard zoet water in te nemen, terwijl men toen toch te maken had met zoutinvloed vanuit zowel zee, als vanuit -Volkerak. Uitgaande van gedempt getij kan, indien met verstand uitgevoerd, nog decennia gewoon zoetwater worden ingenomen. Ondertussen nemen we maatregelen om de hele zoetwatervoorziening anders te gaan regelen. Tijd zat! Ondertussen kan in Krammer-Volkerak een zoet-zoutovergang worden ingesteld in het overgangsgebied Zoommeer/oostelijke OOsterschelde. Deze zone kan de komende decennia langzaam opschuiven naar het noorden. Hoef je ook het KV niet in een klap zout te maken (en er dus een tweede Grevelingen van maken). prima, is lang bestaand besluit. ecologisch waardevol herstel van gradiënt zoet-zout. Nadeel vrijkomen van schadelijke stoffen in bodem zal m.i. meevallen. Geen nadelen doch uitsluitend voordelen. Getuigd van dramatisch slecht bestuur dat dit nog steeds niet geregeld is. Voordeel is meer natuurlijk afvoeren van water met lagere stroomsnelheden. Negatief effect kan zijn op de landbouw. Dit zou een goede bijdrage zijn aan het natuurlijker maken van het riviersysteem ter plaatse. Nadeel is dat de zouttong meer landinwaarts komt (drinkwater, landbouwater) Perfecte oplossing, goedkoop raakt niemand in zijn belangen en levert estuarien herstel in Haringvliet op. goede maar erg veelomvattende oplossing: beinvloedt veel meer dan enkel de erosie. Voordelen: natuurlijke dynamiek kan zich herstellen, ook positief voor de ecologie, de regio kan zich herstellen in een echt estuarium met daaraan gelinkte soortgemeenschappen. Nadeel: zoutindringing leidt tot grote veranderingen in het mogelijke gebruik van het water voor landbouw of drinkwater. Omdat de halve regio hiervoor de waterhuishouding moet aanpassen is dit ook een dure oplossing, waarbij de populatie wederom uitgebreid zal moeten worden meegenomen in het besluitvormingsproces. Laatste nadeel: erosie zal nu plaatsvinden op andere plekken; dit zal goed van tevoren moeten worden onderzocht opdat hier maatregelen voor genomen kunnen worden. Dit kost wederom veel geld en tijd. Al sinds 2000 zouden de sluizen open gaan. En nu is er weer een definitief openingsbesluit in 2018. Dus als men zich hier aan houd, wordt dit plan sowieso uitgevoerd. In combinatie met verbreding een goede zo niet enige goede oplossing. Dus VOOR. Het huidige besluit vind ik halfslachtig. De sluizen gebruiken als stormvloedkering lijkt me voor de natuurlijke dynamiek het beste. Echter ook in de Oosterschelde is sprake van zandhonger en erosie, hier zou extra aandacht aan besteed moeten worden. De natuur in het Haringvliet tot aan de Biesbosch is gebaat bij herstel van het natuurlijke getij. Verzilting ver land inwaarts dient echter scherp in de gaten gehouden moeten worden. Het Haringvliet is niet afgesloten met een dam, maar met spuisluizen om het rivierwater bij hoge afvoeren kwijt te kunnen. Permanent open zetten van de sluizen heeft wat dat betreft dus geen extra nut. Het grote nut van het open zetten van de sluizen is vooral het herstel van 101

het getij in het Haringvlietbekken. Dit blijkt een zeer effectief middel om de erosieproblematiek in het Spui op te lossen, maar de zoutindringing is dan wel een belangrijk punt van aandacht. Het ligt voor de hand dat de huidige zoetwaterinname vanuit het Spui niet meer mogelijk is en dat gebieden langs het Haringvliet zullen verzilten. Een en ander leidt tot grote weerstand vanuit de landbouwsector. Het is (helaas) niet goed omschreven. Het water wordt niet ingezet om verzilting in het Haringvliet landinwaarts tegen te gaan, maar om de estuariene dynamiek op gang te helpen. Het rievierwater stroomt vanzelf naar het Haringvliet als de dam verder open gaat. Het voordeel is dat er voor het getij in het Haringvliet geen water meer vanuit de Nwe waterweg via het Spui hoeft in te stromen. Er wordt dan immers voldoende water via de H'vliet dam aangevoerd. De stroomsnelheden nemen dan sterk af in het Spui en de erosie zal stoppen. Wel kan het dat er erosie ontstaat omdat de stroming de andere kant op gaat bewegen. En omdat de vastere bodemlagen inmiddels weggesleten zijn, kan er dan nog steeds erosie optreden. Lijkt mij de meest eenvoudige oplossing met het minste ruimte gebruik. In de Kil wordt het probleem ook opgelost. Gevaarlijk en risico vol omdat er altijd Zoetwater in de Bernissie naar Brielsemeer moet kunnen stromen ivm met levering Westland ! zout water komt te ver "binnen" Al veel te lang uitgesteld, nu snel zorgen dat die sluizen opengesteld kunnen worden. Is al te vaak en te lang uitgesteld. efecten omgeving moeilijk in te schatten natuur: positief kosten: hoog (doordat je zoetwater tot gouda moet garanderen, kan dat eigenlijk? = risico) Een uitstekende oplossing en bovendien ook te combineren met de vorige. Een voordeel is dat de zoetwateraanvoer sowieso aangepast moet worden vanwege de zeespiegelstijging en dit nu al doen een zogenaamde no-regret-maatregel is. Een ander voordeel is dat het gebruik van de Haringvliet als stormvloedkering niet alleen de optrek van vis zoals zalm mogelijk maakt, maar ook de jonge zalmen weer een goede weg naar de Noordzee wijst. Jonge zalmen verdwalen nu in het stilstaande water van het Haringvliet en vallen daar massaal ten prooi aan snoekbaars, een roofvis van groot stilstaand water. Ik zie weinig nadelen. Zoetwateraanvoer voor de landbouw kan gewoon geregeld worden. Een kwestie van niet zeuren maar doen. Idem voor drinkwater. goede oplossing beter uitwisseling vissen tussen zee en binnenwater verzilting voorkomt algengroei, geen dichtslibben Haringvliet Nadeel relatief voor agrarische ondernemening door verzilting landbouwgrond; vereist aanpassing Voordeel verbeteren kwaliteit water Het open Haringvliet gekoppeld aan de Biesbosch was een schitterend getijdengebied. Of genoemde oplossing economisch en ecologisch haalbaar is betwijfel ik. Bij een open Haringvlietdam geven we een groot zoetwaterbekken op waar geen goed alternatief voor is, dat moet pertinent niet gebeuren. Voordelen; biodiversiteit (aantrekkelijker gebied) en lagere stroomsnelheden. Nadelen; lager water en zout indring. Een kleine opening zou interessante eco ontwikkelingen kunnen geven en verzilting onder controle houden.

Blokkeren van het Spui: Wat vindt u van deze oplossing?

Grootste nadeel is dat je de natuurlijke processen uit het systeem haalt. Die processen zijn fundamenteel voor het karakter en waarde van dit gebied. De afgesloten delta is nu juist een concept dat niet best blijkt te werken... Het Spui zou je misschien kunnen afsluiten, andere noord-zuid verbindingen, zoals de Noord, zijn belangrijk voor de scheepvaart en kunnen niet worden afgesloten. Zij krijgen dan extra water te verwerken en de stroomsnelheden zijn daar eigenlijk al aan de hoge kant voor een veilige scheepvaart. De accommodatieruimte van die verbindingen is dus gering. Nadelen; te ingrijpend en voor de scheepvaart (soms) langere route Rotterdam-Antwerpen, en het wegvallen van een alternatieve scheepvaartroute in geval stremmingen Oude Maas, Dordtsche Kil of Hollandsche Diep. Watervoorziening Westland, wegvallen dynamiek getijden gebied (biodiversiteit). Mogelijk voordeel recreatie. Nadeel is de harde maatregelen bij omliggende rivieren die nadelig kunnen zijn voor ecologie en natuurlijke dynamiek en voor her landschap. Slechte zaak. Geeft wateropvangproblemen in noordelijk rivierengebied. Hoe houdt men het water in het geblokkeerde Spui op orde? Kun je het zoet houden in een verziltende omgeving? Hoe regel je uitwisseling van organismen? Maar snel vergeten. Misschien een idee ,maar er moet zoetwater naar de Bernisse , dan dus afsluiten ten Oosten van Bernissie ( de Beningen) Er zal toch ook water in het Spui ingelaten 102

moeten worden voor de doorvoer naar het Brielse Meer. Stilstaand water is ecologisch niet prettig. Slechte oplossing. Deze oplossing verplaatst het probleem. Belang van recreatie in het gebied is groot. Nadeel is dat je een dode rivierarm krijgt. Ik zie geen voordelen in deze variant. Hele rigoureuze oplossing. De belangrijke recreatie functie vervalt hiermee bijna volledig. Bovendien zal ook hier kostbare landbouwgrond verloren gaan. Ik zie alleen maar nadelen. Deze 'oplossing' is misschien meer t verplaatsen van t probleem. Echter is de oude maas vrij breed en zou dit toch een goede oplossing zijn. Moeilijk om in 1 keer ene mening te geven. Hangt af van balans natuurwinst in Spuigebied en mogelijk verlies van natuur in andere gebieden. In principe positief. Geen goed plan, je zadelt andere rivieren met de erosie op. Goede maatregelen tegen de erosie in het Spui is het beste. Meer ruimte is het tover woord, niet minder. Hierdoor maak je problemen in plaats van ze op te lossen. Dus TEGEN geen goede, het is het verplaatsen van het probleem Geen voorstander van afzetten van rivieren, dat is al genoeg gebeurd. Kan een interessante optie zijn. Voor natuur en vanuit veiligheidsoogpunt aantrekkelijk. heel aardig, maar vraag me af of je de juiste natuur stimuleert. Ik denk dat deze regio meer gebaad is bij getijde-natuur. Wellicht goedkoopste maatregel, maar mogelijk hoge indirecte kosten omdat je omliggende rivieren meer gaat belasten. Ik vind dit een zeer slecht plan. Voordelen: geen. Nadelen: Zelf woon ik aan het Volkerak en weet maar al te goed van de zeer grote waterkwaliteitsproblemen van een groot eutroof zoet meer. Veel plezier met de blauwalgen! Via het Spui heeft het Haringvliet nu nog enig getij. Door dit weg te halen verminderen ook de natuurwaarden in het Haringvliet en zal ook hier de blauwalgproblematiek toenemen. Het beetje getij dat er is zorgt er in ieder geval voor dat nu voedselrijk water over ondiepe oevers heen en weer stroomt, waarbij het opgenomen wordt door algen, wormen en schelpdieren. Deze worden op hun beurt weer gegeten door tal van vogels en vissen. Hoogwater brengt nieuw voedselrijk water naar het bodemleven en laagwater maakt het bodemleven toegankelijk voor steltlopers en andere vogels. Bij stagnant water verdwijnt deze voedselaanvoer en verarmt het bodemleven (zie Volkerak), waardoor ook de vogels en vissen het nakijken hebben. Niet mijn favoriete oplossing, meer nadelen dan voordelen Voordeel: nieuwe mogelijkheid voor een zoetwater plas met een daaraan gelinkt ecosysteem. Meer recreatie mogelijk. Nadeel: waterafvoer wordt opgevoerd bij de omringende rivieren, dit leidt absoluut tot meer erosie in andere gebieden, hogere overstromingsrisico's en noodzaak om dijken elders te verstevigen of te verhogen. Dit is duur en tijdrovend. Er ingrijpend, maar mogelijk de goedkoopste mogelijke oplossing, maar moeilijk voor scheepvaart Voordeel: Het erosieprobleem wordt opgelost in het Spui. Nadeel: Het Spui verliest zijn natuurlijke karakter. De ingrepen in andere watersystemen zullen fors (kunnen) zijn. Wat blijft er dan nog over van het getijdengebied langs Oude Maas, Nieuwe Maas en Noord? Geen goed plan! Lijkt me geen goede oplossing. Het Haringvliet moet dan door de Dordtse Kil gevoed worden. Hier worden de problemen dan veel groter. Klinkt als verschuiven van het probleem naar andere rivieren. Plus dat dan Spui als getijdenrivier verdwijnt. Lijkt me een plan dat nog niet onderzocht is? Er zal goed onderzocht moeten worden wat het effect is op de andere rivieren. Heel slecht plan, want getijdedynamiek verdwijnt en daarmee heel bijzondere biodiversiteit. kan niet overzien, maar vrees een dood en steriel stuk water met algengroei of explosie van watertourisme (waterscooters etc) Dit gaat ten koste van een open verbinding in de ZW-Delta en die willen we juist koesteren. daar komt bij dat het voor het innamepunt bij de Bernisse ongunstig is. Want als het aan de zuidkant van de blokkade-dam ligt, dan wordt het zout bij een open H'vliet en als het aan de noordkant ligt zal zich er zout ophopen dat via de Oude Maas hierheen stroomt. Wel zal de erosie in Spui afnemen, maar daarvoor in de plaats zal de erosie in Oude Maas en Dordtse Kil verder toenemen, want het Haringvliet blijft water aantrekken.

Ground Consolidators: Wat vindt u van deze oplossing?

Lijkt aardige oplossing met vergroten natuurwaarden Voordeel: De kosten zijn (blijkbaar) beperkt. Nadeel: Een technocratische en maakbare oplossing. Geen goed plan, lijkt me. Lijkt me geen goede oplossing. Erosieprobleem ligt mijns inziens op een diepte waar geen plantengroei zal ontstaan. Stroomsnelheid en gebrek aan doorzicht werkt ook belemmerend voor de plantengroei. interessant 103

idee, zou ik meer over willen weten. Uitstekend! Klinkt als een aardige korte termijnoplossing. Het biedt echter geen oplossing voor de erosie van de dieper gelegen bodems. Het slechte doorzicht in de veel te voedselrijke Deltawateren is een groot probleem voor de vestiging van waterplanten. Een goede oplossing zou dan ook diepere bodems moeten beschouwen en filterfeeders zoals diverse soorten schelpdieren mee moeten nemen in de hoop dat die het doorzicht voldoende verbeteren voor een flinke hoeveelheid waterplanten. gezien de sterkte van de stroming in het Spui verwacht ik niet dat vegetatie, zoals riet en biezen, een overlevingskans hebben. Uiteindelijk niet heel duurzaam. Teruggave gronden aan natuur heeft voorkeur. ik ben met deze oplossing onbekend en daardoor geen mening over Moet getest worden door Deltares enz . Op sommige locaties in Nederland wellicht oplossing voor oeverproblemen. Zou in Spui toegepast andere oplossingsrichtingen (estuarien herstel) blokkeren. Dit werkt op de oever, maar het gaat hier om kuilen van 25 m diep in het midden van de rivier. Daar groeit niets. Het is mogelijk wel een interessante optie voor het verstevigen van de oever. Mits deze later met echte vegetatie begroeid kan raken. lijkt goed Klinkt goed. Proberen! Klinkt als eenvoudige oplossing, maar niet duidelijk of dit bestand is tegen sterke stroming van nu. Kan goed onderdeel zijn van pakket van maatregelen. Als je de oevers vastlegt, zal de bodem verder eroderen. Net als bij rivierverruiming leidt dat tot extra weerstandsreductie, daardoor het aantrekken van meer stroming en dus het versterken van de erosieproblematiek. verwachting duur in onderhoud Deeloplossing voor het plan. Nadeel; Bij verontreiniging moeilijkheden bij schoonmaken, meer stoomsnelheid in de vaargeul. Voordeel; biodiversiteit en waterzuivering. Zal niet toereikend zijn. Stroming is het probleem, blijft na afbraak van de GC's te hoog voor stabiele oevers vermoed ik. Ziet er mooi en veel belovend uit. Ben wel van mening dat het gecombineerd moet worden met één of meerdere van de andere oplossingen. Het lijkt mij symptoombestrijding die weinig duurzaam is, als het al werkt. De ruimte is zo beperkt in relatie tot de stroomkracht, dat het de vraag is of hier ook echt een natuurlijke oever zal kunnen ontstaan. Voordeel zou kunnen zijn dat het werkt, maar ik heb daar mijn twijfels bij. Nadeel is dat het geen natuurlijke oplossing bied. Laat het getij weer toe dat werk volgens mij beter. goed plan voordeel: natuurcreatie, versterken van het ecosysteem lokaal, en het creeren van een oeverbeschermingssysteem dat evt meegroeit met waterniveaus en bestand is tegen veranderingen. Het vergroot recreatie en schoonheid van de omgeving en creeert leefgebied voor aantrekkelijke soorten. Kosten-efficiente oplossing. Waterkwaliteit zal verbeteren door aanwezigheid van helofieten. nadeel: onderhoud is nog wel noodzakelijk op sommige delen om bevaarbaarheid mogelijk te houden. Verantwoordelijkheid van onderhoud is niet altijd even duidelijk. Begaanbaarheid van de oever voor recreatie wordt minder met volledige beroeiing. Ook met gebruik van voornamelijk afbreekbaar materiaal zal er waarschijnlijk toch gebruik moeten worden gemaakt van ander, niet afbreekbaar materiaal dat in het natuurlijke systeem zou kunnen komen. Nadeel is dat het on beginsel geen natuurlijke oplossing is en estuariene naruur niet hersteld wordt. Lijkt me een van de beste oplossingen, mede doordat de kosten beperkt blijven. heel leuk. Goedkoopste optie denk ik. Maar geeft verder ook weinig andere meerwaarde. Het onderzoeken waard. Ik vraag me af of deze maatregel hier effectief kan zijn. Lijkt mij van alle hier gegeven oplossingen nog de beste en bovendien het minst ingrijpend in het gebied. Nadeel kan zijn dat er zoveel natuurlijke vegetatie komt dat men in de toekomst moet ingrijpen om dit weer te herstellen c.q. in goede banen te leiden. Kan ik onvoldoene beoordelen op duurzaamheid en kans van slagen in de gegeven situatie (hoge stroomsnelheden, wordt er inderdaad slib in gevangen, etc.)

Wilgen matrassen: Wat vindt u van deze oplossing?

Dit is een andere manier om de oevers vast te leggen, met hetzelfde bezwaar als de vorige. Ook uitstekend. Ook deze oplossing blokkeert "groot denken". We blijven hangen in traditionele maatregelen "tegen iets".Deze oplossing kan ook en zal als voordeel hebben dat doordat er natuurlijke materialen gebruikt worden ook veel vogels en andere dieren gebruik maken van de versterking. Nadeel is dat het niet bepaald fraai oogt. Althans de eerste jaren niet. Lijkt me geen oplossing. Is m.i. niet bestand tegen het geweld van stroming in de rivier. wellicht de leukste en goedkoopste optie, met 104

er uiteindelijk een soort zone met wigen ontstaan? dat zou wel leuk zijn en bijdragen aan de natuurwaarde Voordeel; net als steenstorten, duurzaam en combinatie mogelijkheden. Nadeel; de stroomsnelheid neemt niet af kostbaar. Uitsluitend voordelen. We hebben voldoende wilgen in de Hoeksche Waard om deze wilgenmatrassen te maken. Voordeel: Het sluit aan op traditionele vormen van oeververdediging. Nadeel: Het houdt te weinig rekening met de ruimte die water nodig heeft: het fixeert te veel. Geen goed plan. Zie consolidatie Werken met wilgen lijkt me natuurlijker dan met de GC's en heeft dus voorkeur wat mij betreft. Of het hier kan werken kan ik ook niet niet beoordelen. geen mening Evenals het voorgaande, het onderzoeken waard. oude methode , probleem levensduur Ziet er mooi en veel belovend uit. Ben wel van mening dat het gecombineerd moet worden met één of meerdere van de andere oplossingen. Praktische en goedkope oplossing als onderdeel van pakket van maatregelen. Kan ook helpen met biodiversiteit te vergroten. Een slechte oplossing, want het pakt de oorzaak niet aan en is daarom altijd tijdelijk. Bovendien is het geen duurzame oplossing voor de toekomst in die zin dat het geen enkele stap is in de richting van een klimaatbestendige Delta. Het kan echter wel een (klein) onderdeel zijn van een andere, meer structurele oplossing. interessant Ook een mooie oplossing. Zo blijft de huidige omgeving behouden. goed Eigenlijk het zelfde als bij vraag 4. Een onnatuurlijke, en wellicht, tijdelijke oplossing. hoe lang is de levensduur? Geen oplossing die goed landschappelijk kan worden ingepast. niet duurzaam genoeg, veel 'verstoring' langs de oevers. Duurzamere 'ruimte voor de rivier' heeft de voorkeur. Zie vorige vraag.ecologisch gezien aantrekkelijk een veelgebruikte methode die zijn succes over de jaren heeft bewezen. voordeel: gebruik van natuurlijke materialen ,kan lokaal gemaakt worden, creatie van nieuw leefgebied, de matrassen zullen begroeid raken met helofyten zoals in oplossing 4, ecologische voordelen zullen na verloop van tijd gelijk zijn aan die van de vorige oplossing. goed te combineren met andere oplossingen. waterbouwers hebben al veel ervaring met deze methode. nadeel: tijdrovend om de matten te maken, veel handelingen, redelijk duur. onderhoud is beperkt nodig. Klinkt ook goed. RIjshouten zinkstukken zijn vanouds bekend. Indien stroomsnelheid langs de oever permanent kan worden gereduceerd door extra ruwheid is er een oplossing voor de ondiepe oever. NB stroomsnelheid aan de bodem in in de vaargeul nemen dan toe. Behoud van erosie probleem. Een charmante oplossing, maar werkt alleen op de oevers, terwijl het probleem zich vooral in de diepte bevindt.

Oplossingen

“Waarom heeft u de oplossingen op deze volgorde ingedeeld?”

Wil graag zo dicht mogelijk blijven bij de natuurlijke ordening in een rivierdelta, omdat dat het beste toekomstperspectief biedt, zowel op het gebied van beheerskosten, als ook de ecologische- en belevingswaarden. Ik ben altijd een warm voorstander geweest om de Haringvlietdam open te stellen. De biodiversiteit zou zich dan pas weer kunnen herstellen na de ecologische ramp na de afsluiting. Spui is/wordt nu bestort, zo laten lijkt me het beste Een open Haringvliet is het herstel van de situatie waarin nog geen erosie optrad. Daarbij heeft het veel andere voordelen. De andere oplossingen lossen volgens mij het probleem niet op, daarom is voor mij het zo laten van hetSpui de tweede. Maar ik laat me graag verrassen als het anders is. De opties voor het versterken van de oever spreken me daarna het meest aan. Maar ik ben bang dat ze onvoldoende opleveren. In het kader van de tafel Ecologie van het DP Rijnmond Drechtsteden zijn ook ideeën verzameld voor het verbeteren van de vooroevers, door wilgengroei en het weer laten opslibben. Misschien is dat ook nog een optie. Meeste voordeel op lange termijn. 1 wellicht meest duurzame en onderhoudsvrijse oplossing met meeste rendement voor andere functies 2,4,5 zijn geen complete oplossingen 6 kost veel 105

onderhoud, 2,3,4 dragen bij aan reductie van deze kosten. Weet niet waar de omslag zit naar compleet nieuw systeem zoals 3 of 1 We zijn goed in staat om de erosie in de hand te houden Al aangegeven bij de verschillende oplossingen kosten buiten beschouwing gelaten... Het meest waardevol is de oplossing die de meeste typen meerwaarden in de regio brengt (waar de regio een tekort aan zou hebben). Er moet iets gebeuren, maar de Oorzaak moet eerst gevonden worden IK denk dat een zo natuurlijk mogelijke rivier (open sluizen en breed) de beste oplossing is voor mens, dier, natuur en zeer duurzaam. het is een uitgelezen kans om natuurherstel te bevorderen, gezocht moet worden naar de optie met meeste natuurwinst. Open Haringvliet zou prachtig zijn maar misschien niet haalbaar dus andere oplossingen; kan eigenlijk nauwelijks kiezen zonder uitgebreidere informatie. De beste twee oplossingen zijn de negatieve gevolgen voor de betrokken partijen geminimaliseerd. Daarnaast ziet het er uit als een duurzame oplossing. geef voorkeur aan ecologische oplossingen Openstellen van de sluizen brengt de natuurlijke situatie (voor een deel) terug. Zie studie Tom Smits en commentaren in het voorgaande Landschappelijk goed inoasbaar en dan in combinatie met open Haringvliet Opties 1 en 2 kunnen ook gecombineerd worden, waarbij 4 en 5 ingezet worden waar nodig. Deze combinatie - maar ook oplossingen 1 en 2 afzonderlijk - maakt een zeer klimaatbestendige en natuurrijke Delta, waarin het goed wonen, werken en recreëren is. Geeft herstel van natuurlijke sedimentatieprocessen. Lokaal kan eventueel gewerkt worden met consolidators en wilgenmatten. Bij een open Haringvlietdam wordt de getijdendynamiek hersteld, wat voor de veiligheid als voordeel heeft, dat het water niet constant aan hetzelfde stuk ''vreet''. Voor de natuur houdt dit idee herstel in van de oppervlakte getijdengebied én van de zoet-zoutgradiënt. De beste oplossing lijkt me een combinatie van Oplossing 1 en 2: De dam open én een bredere rivier. Afsluiten van het Spui en zo volledig weghalen van natuurlijke dynamiek in het gebied is voor mij de slechtste oplossing. Aanpassingen in dynamiek in bijv. de Haringvliet, Grevelingen en Oosterschelde hebben ons geleerd dat dit in eerste instantie aantrekkelijk kan lijken en ook voordeel kan opleveren, maar dat de ontwikkeling van het systeem tot een ander systeem onverwachte en onaantrekkelijke wendingen kan nemen. Door dynamiek weg te halen kan er geen gebruik meer worden gemaakt van de natuurlijke functies en diensten die een riviersysteem heeft (buffer functie voor overstromingen, habitat voor euryhaline soorten) en wordt de druk op omringende systemen opgevoerd. Meer natuurlijke maatregelen, waarbij er wordt gestreefd naar erosiereductie door natuurontwikkeling aan de voet van de dijk, hebben daarom altijd mijn voorkeur, omdat deze in grote mate zichzelf onderhouden, automatisch ook meerdere diensten en functies leveren (waterkwaliteit, luchtkwaliteit, habitat) en aantrekkelijk zijn. Zie eerdere antwoorden

Wilt u hier verder nog iets over kwijt?

De vraagstelling is teveel vanuit de "groene" hoek opgesteld. Gebruik maken van afsluitbaar binnendijks gebied; Bernisse en Vlietproject. nee Bij een open dam kan er evt zandhonger ontstaan. Onderzocht moet worden hoe het bij een open dam gaat met de sedimentaanvoer vanuit zee en vanuit het achterland. Graag de alternatieven in een onderzoek verder uitwerken Het is nog niet bekend wat de oorzaak is, men gaat (is) onderzoeken , dan kunnen daar gerichte maatregelen tegen genomen worden. De alternatieven zijn niet gelijkwaardig. Is dit wel uitputtend? Zijn er KRW-kansen?

Indien werkzaam, voor welk bedrijf werkt u? (niet verplicht) stroming bouwadviseur nvt Deltares ARK/ Wereld Natuur Fonds NBC Klein Profijt Vereniging Psyient Nederland ARK Natuurontwikkeling Eigen agrarisch bedrijf

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Heeft u nog vragen, op- of aanmerkingen naar aanleiding van dit onderzoek?

Summiere ideeën op http://www.swaneblake.nl/images/klimaat_en_swaneblake_2014.pdf Leuke vragenlijst, succes met jullie project! Hoop dat versneld tot ontwikkeling kan worden overgegaan. In de jaren 90 presenteerden zowel RWS als Natuurmonumenten grootschalige plannen rond het Spui. Ook WNF ( en Havenbedrijf Rotterdam) blazen hun partijtje mee. Met Waterschap de Hollandse Delta heb ik een accoord bereikt voor het ontpolderen van anderhalve hectare verdroogd gors (2015?) als compensatie voor dijkversterking Spui-oost. Hoera! Zie eerdere commentaren en opmerkingen veel succes!

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E. Outcomes of the Multi-Criteria Analysis

Socio- Ecological Land Use economic Costs Multi-Criteria Analysis value

criteria

use

-

intrusion

-

costs

Safety

Nature

Housing

Esthetics

economc criteria economc

Navigation

Recreation

Agriculture

Respondent

-

Local support Local

Drinking water Drinking

Ecological value Ecological

Diverse land Diverse

Landscape value Landscape

Installation costs Installation

Maintenace costs Maintenace

Stop salt Stop

Increase biodiversity Increase

Longevity of solution of Longevity

Draagkracht van plan van Draagkracht

Restore tidal dyanmics tidal Restore

Socio Prevent Erosion/promote sedimentation Erosion/promote Prevent 1 30 30 10 30 50 25 25 30 30 40 50 25 25 25 50 25 2 20 20 10 10 20 20 25 25 25 25 15 20 15 50 33 33 33 5 10 50 10 25 3 40 30 30 30 20 30 20 40 20 0 40 20 50 30 10 20 20 20 30 4 10 10 10 0 60 10 10 40 40 10 40 30 30 40 50 10 0 40 50 10 5 25 25 50 25 25 50 25 50 25 15 70 15 20 20 50 10 6 100 20 10 70 50 50 30 40 30 100 7 30 20 20 30 20 20 30 30 30 20 30 20 30 60 10 20 20 20 20 20 8 5 20 20 20 35 25 25 25 25 40 10 20 30 10 80 10 15 5 50 25 5 9 20 0 30 20 25 5 15 25 40 20 5 50 10 35 30 50 20 15 10 40 20 15 10 10 20 10 15 20 25 15 35 30 20 25 25 25 25 30 40 30 20 20 25 25 10 11 40 40 20 20 20 60 80 20 25 50 25 60 40 12 30 30 20 10 5 5 10 70 10 10 60 15 15 10 100 5 70 20 5 13 20 10 20 25 25 25 25 25 25 25 25 25 25 50 50 25 50 25 14 10 30 20 30 25 25 50 50 50 60 40 100 15 30 5 15 5 30 15 20 30 30 20 30 10 30 30 20 50 30 0 10 50 30 10 16 30 40 30 50 50 30 70 40 60 0 10 50 20 20 17 25 15 20 5 30 5 25 20 30 25 35 15 30 20 35 40 25 15 20 25 25 15 18 20 5 5 0 60 10 15 20 60 5 40 15 30 15 10 70 20 15 10 10 60 5 19 5 10 5 0 70 10 25 20 50 5 50 0 50 0 35 30 35 10 100 10 70 10 20 10 20 10 40 20 10 20 50 20 30 60 10 25 50 25 10 30 50 10 21 20 20 15 10 15 20 25 25 25 25 25 20 15 40 20 40 40 15 15 30 20 20 22 10 10 70 10 20 20 60 60 40 10 60 30 55 45 23 10 0 0 0 90 0 20 80 20 80 50 50 100 24 20 20 10 30 20 20 20 40 20 40 10 25 25 32 35 33 5 5 40 20 30 25 20 10 20 0 40 10 30 20 40 10 30 10 30 30 10 80 10 10 10 30 40 10 26 10 30 10 20 20 10 25 25 25 25 25 25 25 25 25 50 25 10 10 30 30 20 27 25 10 10 15 20 20 25 25 30 20 25 0 75 0 20 50 30 20 25 10 25 10 28 10 10 30 10 10 30 20 15 15 50 0 50 0 50 25 25 50 90 10 29 10 40 10 10 30 10 40 20 30 30 30 10 30 20 30 50 10 30 40 10 10 30 10 10 40 20 10 10 20 30 20 30 30 35 30 5 30 30 40 20 20 20 10 30

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